Leak test

could be the matterial soaking up water (nothing is totally inpermeable) or even the water in the cockpit!!!

hen it come to unscrewing the bungs after sailing you can hear the air escaping

this shud mean the hull is water tight or as near as dam it

You've got to remember the hull panels flex in and out as you hit the waves and so some of the time the pressure is less inside than out.  A tiny leak will be alternatively sucking and blowing.  If there is water around the leak then water will get sucked in but run to the lowest point inside so that air get expelled on the next blow.  You then lift the boat out of the water and the air inside warms and expands.  If you then open the bung you release that pressure but find a small amount of water in the hull.  Its common and not to be worried about - just make sure you sponge it out, especially if its a wooden hull and especially if its likely to freeze overnight.

Cheers everyone whos put their point in  , weve got a feelin its where the spaceframe goes through the hull at one point, once sealed hopefully itll wont leak!! we had noticed that the boat only take the water in when its on its side , if we race and dont capsize it doesnt leak!