Hi Peter,
You will need to capsize the boat in a controlled environment, either in shallow water near a shoreline or in the presence of a support vessel ( both sides independently ) and visually note how far the boat sits out of the water on its side. Right the boat, empty it of water and then get it to a position where you can then check the buoyancy tanks for water ingress. You haven’t stated how old the boat is so I can only guess that the buoyancy you have comprises of two side chambers and one bow chamber?
If the buoyancy is working as expected then when the boat is on its side there should be very little of the cockpit that is submerged, perhaps just a little deeper than the width of the freeboard. To give an idea of what to expect – after a capsize ( in sea conditions ) in my 1973 GP14, the water in the boat once upright is only ankle deep.
You may want to check first though that the bow buoyancy tank isn’t taking in water via the keel band fixing screws during a normal sailing session. If it does, then get that sorted first.
I hope that helps,
Simon.