A very belated reply to my original query.
The space under the cockpit floor turned out, of course, to be a sealed buoyancy tank and, in my boat, filled with foam. I had to cut a handhole from the locker to get access. The nuts on the U-bolt were simple ones, with no attempt to lock them, although they lasted nearly 20 years before coming loose.
I have since fitted a traveller on the bridge deck, using 30mm genoa track, mainly to keep costs down. It is only as wide as the flat section of that part of the deck, nearly 2ft. With it, I can position the boom as required when beating, but a more sophisticated traveller is required for spilling in gusts - I still have to use the mainsheet for this. The main reason for the change was the main sheet block kept hitting me in the face when gybing, and the new traveller has solved that problem.
The leak was from the windows. I had used some domestic mastic as sealant and it was not good enough. I have since replaced both windows with new acrylic sheet, the first about a year ago using Sikaflex sealant, and the second a couple of months ago using a Dow marine silicone recommended by the local boat yard. Both are watertight so far. I was advised to use pan head screws rather than the orignal countersunk ones, as they are less likely to split the plastic - seems a good idea to me, as the original windows were starting to split.