I've got a '76 Islander 28, I'm 6'4", 240 and have figured out a way to contort myself to fit down in the port cockpit locker. By now (thread started 6 months ago) you've probably figured it out too. Having replaced the cockpit drain hoses, backstay chainplates, ignition wiring for control panel, etc...I know it's tight down there for us big guys. For the record, this is how I get aft bilge access:
1) start by taking batteries & any other stowage out of the cockpit locker.
2) place an old sleeping bag/blanket down in the bottom for some cushioning & to keep from getting wedged in the angular pit formed between the bottom of the vertical plywood engine compartment panel and the hull.
3) step in to the cockpit locker, then transition to down on your knees (facing towards the bow)
4) now is the contortionist move - getting under the fuel fill hose. Fortunately mine is JUST long enough to stretch a bit above my head as I unfold backwards. You're just going to have to feel around best here as you can, but it's doable. Pushing your feet out and transitioning to your butt is all in this move. Just watch that you don't push on the wiring panel.
5) once you get this move done, it's just a matter of pulling/pushing to get access to things in the aft bilge.
Getting out is actually easier, in my opinion. Of course, that depends on how much time you spend down there. Good luck. ~Joel