Been through similar problems with my ancient mainsail too. Some useful thoughts and observations:
1. As others have mentioned, a shrunken boltrope may be a problem. We eased mine out a couple of inches and re-stitched it and this helped.
2. Is the masthead sheave in good nick and running freely? After many years use the pin that the it runs on wears thinner so that the sheave binds or seizes as soon as you put a heavy load on it – like you do when the sail gets towards the top of the mast. Mine was seizing up when the sail was 2 or 3 inches short. I removed the sheave box from the mast and was able to renew the pivot pin. Sail now goes all the way up with very little effort. If you have to pull really hard to get the sail to go up the mast generally then have a very careful look at the sheave boxes.
3. Compared to more modern dinghies Wayfarer booms are very low. I went down the route of raking the mast further forwards but all I succeeded in doing was creating savage lee helm which made it virtually unmanageable in strong winds! Since then I re-raked the mast as per the measurements in the Wayfarer book and just live with the low boom – it’s character building!
Windermere is indeed a v. beautiful place to sail, though blessed with unbelievably fluky winds due to the surrounding hills, especially at the southern end where we’re based.