Hi Seb, emailed this to you but can now post it so here it is for anyone else!
There is an inner and outer drum with a flat plate on either side, flat plate has a hole in to allow air in or out with a cover over. The ones in the photo have a gauze as well as a flashback arrester. The ball (red) is just smaller than the hole in the plate and the distance between side plates,so if it floats it'll block the hole in the top of the inner drum (fitted with gasket, green) The bottom edge of the inner drum is exactly at the same level as the bottom of the hole in the side plates to allow water to drain out.
Grey is engine box / lid / deck, fetching pink / purple is louvred plate
First pic is steel vent as used on ships, 2nd is adapted for back of P22 enginebox. (Side elevation) All right So I've uploaded them the wrong way round...