If you haven't done so already give the lever a good blast with water and then a liberal spray of silicone just to be sure it is debris free.
Does the washer under the lever slot screw head move? If not you will need to back off this screw as KB said.
An offset screwdriver or a screwdriver bit held with mini locking pliers should give access without dismantling.
Thanks for all of your suggestions. I have found and fixed the problem. The issue was really one of poor maintenance.
There was a bit of debris, not a lot, but enough to reduce tolerances. But the real problem is that I hadn't tightened the bolts around the mast bearing plate in quite a while. They were all a turn and a half loose. This was enough to interfere with the motion of the lever (the problem I was having last fall but never got around to worrying about.
) So over the winter, the nut actually sort of "jumped the track". In other words, there is a slot in the lever that allows travel and the nut that receives the mast bearing bolt somehow got to the wrong side of the slot. So the lever couldn't be pulled and the retaining mechanism was stuck on the inside of the mast well. It wasn't too difficult to cajole the lever back to the other side at which point the nut is back inside the slot. Everything works perfectly now.
I really must get better at maintenance. It's just that I'd rather be sailing. And this boat spoils me by being so trouble free.
Thanks again for all of your advice!! I would have been taking the whole thing apart and probably messed something up in the process.