Thanks for your interest Malcolm.
The lid carries a channel with a neoprene tube insert as a sealing strip that, as you say, fits onto a moulded locating frame set into the deck around the hatch with a raised ridge that fits into the channel and seal; all fastened by two locating lugs at the rear and a sprung clip at the front.
At present it does tend to let water in when a gust of wind dips the 'gunwales awash' but I'll follow up the idea of a smear of vaseline on the seal and a drain bung in the hull – simple solutions that may well be the best! I also reckon I could position and waterproof the receiver etc. more defensively.
Interesting that the instructions suggest that after fitting the lid self adhesive tape should be applied over the join; a fiddly and messy job at the pondside. I sometimes look enviously at the 'slide and lock' lid on a colleague's Robbe Estelle, but perhaps you only get what you pay for!
Thanks again, Howard.