If you are going well offshore you need to be able to fix and make things, the same goes for extended cruises in wilderness areas, there are some chandleries in the Scottish Islands but they are few and far between and most are orientated more towards the fishing fleets than passing yachts. I have split this list into categories but there is a lot of overlap.

Some "How to" books will be useful, especially if not familiar with diesel engines, electrics etc.

This was largely converted from my inventory list in Excel which originated from an "needed" list, I am sure I have missed things I have and things I should have!

Bosuns and Riggers Stores

In addition to ground tackle and spares, see Anchors, Drogues & Emergency Steering I carry all of the components for pre-planned emergency equipment such as for emergency steering, lying to the drogue and setting the storm jib plus.

Silicone sealant & glue.
Bodge (Duct / Gaffer) tape.
A West System Glass Fibre Repair Kit Pack.
Epoxy filler.
Epoxy glue.
Superglue, gel is usually best.
PVA glue, liquid, not spray which sets off gas alarms.
Wirerope grips.
Length of rigging wire (My long piece is Dyneema rather than wire, see under cordage), I also have a couple of short wire stops with eyes each end that have come my way over the years.
Big selection of fastenings (bolts, machine screws, wood screws, self tappers etc. pan head, countersunk and hex, plain and spring washers, plain and Nylock nuts ).
Selection of split pins, split rings, clevis pins, circlips, etc.
Lengths of stainless threaded bar, large washers and nuts. 8mm is likely to be the most useful.
Spare shackles (including a big and a very big galvanised one), thimbles (plastic & steel) and Carabiners.
Monel Seizing Wire.
Snatch block.
Spare swivel block big enough for the largest running rigging.
U bolt long enough to go through the deck.
Marine ply, at least enough to make a backing pad for the above, also useful for patching.
Conforming tape.
Anti-chafe materials.
Spare Winch & Bilge pump handles kept below.
Winch Oil.
Winch service spares, best to have a service kit for each winch type and size on board.
Re-arming kit for life jackets (or spare life jackets above maximum crew size).
Spare dry cell batteries of all types in use.
Silicone spray.
Wood bungs and wedges.
Jubilee clips in various sizes, include at least one very big one, very handy for securing something that is broken or needs holding in place whilst being fixed, as are...
Cable ties, get a good supply including lots of big ones. Some chandlers sell them individually and they will cost a fortune, go along to Halfords where they sell a tub containing 600 assorted ones for < £12 or get a selection on ebay. I may have saved my boat using these, when the engine sea cock failed a wooden plug would not fit because it was a gate valve, I bound it with cloth and secured it with lots of cable ties, that completely stopped the rush of water whilst I organised a tow (I clearly couldn't use the engine) from the mooring to be hoisted out.
"Mousing" line at least long enough to go to the mast head and back - also needed when the halyards are removed for cleaning over winter.
Small plunger to clear the sink or Loo.
Bosun's chair, mast climbing kit if single handed.
Repair kit for the inflatable.
Wetsuit, mask, snorkel and fins - you may need to free rope from the prop, do some basic scrubbing, check a mooring or if the water is not too deep you may be able to free a snagged anchor. A water sports helmet is a good idea - having several tonnes of boat come down on your head is not nice, after the first time I got the helmet, it could also be useful on deck in an emergency if bits of the rig are flying about.

Cordage

Every boat should have plenty of spare rope, you never know when or what you may need. Ropes can get chafed from "normal" use, cut away in an emergency, wrapped around a propeller or simply lost overboard. I have been fairly fortunate with Sancerre, in almost 14K NM I have only:

Lost 1 spinnaker guy that came unclipped after a bad broach, the sheet kinked at a block and the sail flogged whilst I freed things and the guy disappeared - its not a good idea to put a stopper knot in a spinnaker sheet or guy in case you have to let it fly, in the case of the sheets I break that rule single handed but compensate by having extra long ones. Fortunately the sail was not harmed, probably because I had specified a heavier than normal cloth (1.25 oz) for a big light wind kite to allow for this sort of thing happening single handed.
Left one dock line behind leaving Blythe in the middle of a pitch black night, it was an extra line I had put out to midships to help hold the boat off of the pontoon during a severe gale and I foregot about it.
A couple of chafed dock lines, they are always vulnerable - repurposed into bridles etc.
Chafed jib halyard, replaced with wire as even 8mm Dyneema caught on the spinnaker sheave or more likely an over long self tapper.
One broken wire Halyard due to a wrap because whoever installed the roller furling jib did not put in a halyard diverter which it turns out is needed on the A9m.

Back in the day (Morgan Cup 1978) racing the half tonner Green Dragon in pretty wild conditions, surfing at up to 14 knots through the water (no GPS back then) we broke two wire spinnaker halyards (they had to be wire as they doubled up as jib halyards and this was before Dyneema etc), Peter Bruce in "The Goodies" did the same. I was running the foredeck and was cream crackered after that one!

So you need to be prepared to replace just about any piece of rope on the boat and have enough for emergencies such as setting up a jury rig or lashing down part of the boat that has come adrift.

In addition to lines described under "Anchors, Drogues & Emergency Steering", handy billies etc., I carry the following spares made up (all hardware fitted and ends properly spliced or whipped) as appropriate:

10mm Spinnaker / cruising chute sheet.
Old 12mm Spinnaker Guy.
12mm headsail sheet.
Old mainsail halyard.
Old mainsheet.
Spare standing part of the main boom preventer (can be rigged as a second if things were really bad so after a gybe I would not need to go to the mast to reset).
10mm general purpose foredeck line (snap shackle at one end).
Lengths of 8mm braid on braid line as used for reefing pennants etc.
Length of 10mm braid on braid line just in case.
1 x 14mm and 1 x 12mm spare dock line (in addition to the 4 kept in the wet locker and two long ones for high walls and locks. These double as emergency lashing lines and I also use them to lash the boat to piles when drying out.
Several short odd lengths of braid on braid and 3 strand nylon in smaller diameters.
Plenty of "small stuff".
Spare shock cord & hooks / clips to fit.

For real emergencies I also have:

c 25m of 8mm Dyneema, a repurposed jib halyard when it was replaced with wire.
The topping lift is also 8mm Dynema, way over the top for its day job but strong enough to take over from the main halyard or even the back stay or a shroud at a pinch.
I have also left my original Dyneema inner forestay in place although I now have one further forward at the base going to the mast head, this can be used for the storm jib (perhaps if changing down from the heavy weather jib) or to replace or back up a forward lower shroud, these failed on the prototype A9m during the OSTAR, they are 1mm bigger on production boats but end fittings can always fail.

This can get expensive, but to an extent it can be mitigated. Consider changing some running rigging while it still has life in it, you get some nice looking comfortable new running rigging and a serviceable spare but remember that braided rope that has been heavily loaded is hard or even impossible to splice.

Learn to do your own splicing and whipping, a good thing for several reasons, then you can more easily buy and use reel ends, Jimmy Green on-line discount these by 40% and most chandlers have a bin of them - have a rummage as you pass. If you are lucky you may find a bin end that will meet your needs perhaps with some left over, you may have to forego some of the traditional colour coding but I'm not sure how many still use or are even aware of that.

Discounts can be had, at least from Jimmy Green, for 50 or 100 metre lengths, the discount on the former is modest but on a 100m its around 30%. It is certainly worth considering this option especially for anchor rode and dock lines.

Sailmakers Stores

Set of splicing fids / needles.
Sailmakers needles, palm and wax.
An awl can be useful for heavy sailcloth or multiple layers.
Selection of sailmakers thread.
Whipping twine in 2 or 3 sizes, a selection of colours so you can match the rope or marker makes the job look more professional.
Sail repair tape.
Piece of sail cloth for patches, reinforcement or chafe protection. Tip: glue in place then stitch.
Spare batten(s) at least one long one, it can always be cut down. May not be practical for a fully battened sail.
If the main is fully battened with patent inner ends, spare rollers etc may be good to have.
Spare sail sliders & attachments and hanks if used.
Some spare tell-tells may come in useful.
Webbing tape.
Spare sail ties.

Engineers Stores

Assumes inboard diesel.

Fuel treatment (and use it!).
At least 1 new alternator belt (and optionally some old ones). Toothed premium belts are best and essential if you have an external regulator or uprated alternator.
Service kit(s) for all engines (including all consumables for all services that might come due, minimum one oil and filter change).
Engine manual(s), preferable including a full workshop / service manual. I have service & operators manuals in PDF form.
Engine oil extractor.
Funnel for refilling oil if required (I use a Hopkins Tight Spot Flexible Funnel which makes filling with limited head room much easier).
Several spare elements and O rings for the fuel pre-filter in case of diesel bug.
Primary oil filters and O rings.
Prop / shaft anode(s).
Water Pump Belt(s)
Water pump Impellers & gaskets at least 2 of each type and size, preferably new.
Length of premium coolant hose, my Yanmar 2GM20 uses 1/2 inch car heater hose. I didn't have any when needed although I worked round the problem.
Engine Oil for all engines, Gearbox oil, Marine Grease.
Vaseline.
PTFE tape
Optional but preferable:

Spare fuel injector.
Stern gland packing material.
If the engine is an unusual type or very old, or if you are in far foreign parts, a spare head gasket and others required during its replacement is a good idea, a good marine engineer can replace a head gasket on most engines but needs these which may not be available.

Spares for the outboard including shear pins and spare kill cords

Principle tools

My 1/4" socket set, the long reach sockets
can be very useful. Often a 3/8 or 1/2" will be
too big to get at a critical bolt. It can also be
used with hex bits (see below).

Medium quality 3/8" (c 8mm - c 22mm) & 1/4" (c 4mm - c 14mm) Metric Socket Sets, I have Draper Chrome Vanadium sets which are more than adequate. A long extension and long lever is useful, mine are 1/2" with a converter, needed for removing the starter motor on the Yanmar and probably other things,
This bit set will fit most things, use with
a good quality driver and with the supplied
converter with the 1/4" socket set. Unlike
the socket set these will rust so spray with
WD40, gun oil or similar.
Ratchet Screwdrivers + Bit set, a good quality stubby is best.
Big screwdriver with square shaft to take a spanner or mole grips.
Combo ring & open ended metric spanner set.
Open ended spanner to fit injectors (a set may not include the right one, mine didn't).
On older boats some Whitworth spanners may be required (e.g. for old Blakes bronze seacocks).
Older British and some American engines and gear may require AF sockets and spanners.
Adjustable spanners & pipe wrenches, including those needed for the stern gland.
Pliers, long nose pliers, side cutters, metal sheers.
Mole grips.
Bolt croppers. A battery driven grinder would be better.
Battery Power drill + Cobalt bits or similar that will cope with stainless steel.
Lump hammer.
1" cold chisel.
Wood chisel.
Rubber hammer.
Nail bar, use as a lever, e.g. for tightening the alternator belt.
Strap wrench (especially for removal of oil filters).
Stanley knife or similar, preferably with saw blades as well.
Round and flat or half round files.
Wire brush, wire wool and a brass brush can also be useful.
Wet and dry paper or emery cloth.
Large and small hacksaw with spare blades.
Steel or plastic mirror. Tip a GoPro or small camera can be useful to see what is going on, a waterproof one even more so if you get something round the prop or other underwater problem.
Good rechargeable (via USB or 12V) LED work light.,
Wire coat hanger or strong wire for use as a hook.
Penetrating oil (Liquid Wrench is very good, note that some sprays can set off gas alarms, this one doesn't).
WD 40 or similar.
3 in 1 oil or similar.
Locktite thread locker (medium or light grip).
A small battery operated pump is useful for getting liquids out of odd spots.

Electricians Stores and Tools

Electrical tape
Spare fuses of all types and ratings in use.
Spare bulbs.
General electrical spares, connectors, crimping tool etc.
Electrical meter.
12 / 24 volt or gas powered soldering iron, fluxed solder.
Electricians screwdriver, side cutters, stripper if not integral to the crimping tool and you are not comfortable doing it in other ways.
Cable / wire.
Chocolate block.
Manuals for all instruments etc. Most can be downloaded in PDF form from the makers web sites.

General Stores

Water purification tabs.
Tender tube repair kits.
Oil lamp wick, spare glass.
Canvas cleaner.
Spray cleaner.

SV Sancerre

Threads & Pages

Bosun's, Engineers & Sailmakers stores and tools for a long cruise