Many thanks to Matt, Kingy and Tim.

I decided to take Kingy's advice and pump the hulls up with air. The bike pump idea didn't work so used an air compressor - very carefully. The centreboard cases on Boyer boats are very thin and flexible, so I was able to look down the slot while inflating and watch the sides of the cases bulge inwards as the air pressure increased.

The port hull would only hold pressure for about 20 seconds but the starboard hull would hold for about two minutes. So it would seem that the starboard hull only has a minor leak, which I find perplexing seeing as it had about four litres of water in it yesterday.

We did the soapy water test on the pintles, and found the port top pintle was leaking, but nothing on the other three. A leaky top pintle would explain why the hull wouldn't hold pressure for long, but it doesn't explain how four litres of water got in there yesterday in calm conditions.

We removed all four pintles and re-siliconed them, but I am not at all confident that we have fixed the leaks. It would appear that the leaks are one way; from out-to-in but not from in-to-out. Both of the bottom pintles' bolts were tight and the pintles weren't moving at all so I think it is unlikely that they were leaking.