The aperture is rectangular inside so the wood bungs you are recommended to have and I had to hand, would not work but I managed to make up a pad (one handed whilst the other was over the leak) and fix it over the hole with cable ties.
With no cooling water I had no engine, so I called Deacon's who came 2 miles down river to tow me up to be lifted out and a couple of hours after the failure she was safe and dry.
The pad held OK so I only had a few gallons of water in the boat that the auto bilge pump coped with easily.
A new composite sea cock will replace it, also the one for the sink which is also a gate valve, £48 for each valve and a new skin fitting, rather more to have it fitted (Edit: as it happened a LOT more, follow the thread for more). The (comprehensive) insurance will not cover the valve replacement as it was due to corrosion but does cover the tow and lift out.
You have been warned!