I spent a few hours today epoxying Jim's dinghy along the bottom. I found the source of the leak, it was where all the epoxy kept disappearing! Once it had hardened, I turned the dinghy over and sure enough, epoxy had leaked through the hull. It looks as though the hull had a weak spot during manufacture, because there was plenty of glassfibre mat there, just no resin.
Anyway it should be good to go tomorrow after the epoxy has had 24 hours to set.
I also get to pick up my own dinghy tomorrow. Its just an old sailing boat hull, but as long as its got somewhere for the oars and it stays afloat, I don't care, especially as it only cost £16. Yes, that does mean technically the dinghy cost more than the yacht.