geardad said:531 is what that decal says, alright!
As for your other suggestions, like repacking the bearings, should I attempt this if I've never done the procedure before? I'd be concerned about doing it right, accurately spotting things needing replacing, re-assembling properly.
move RD closer to drop out? meaning, closer to the closed end or to the open end? the dropouts have those adjusting screws....
geardad
Repacking bearings isn't rocket science. Using appropriate tools, dismantle the bearing, soak the parts in kerosene or your favorite non-flammable solvent, dry, replace damaged parts, repack and reassemble, and adjust by feel--there should be no play but the bearing shouldn't bind, either. Don't worry if you overtighten a bit, just loosen and try again. When it all feels good, batten down the lockrings and test again.
There are numerous inexpensive books on bike repair that can show you the whole process. This is the way I started on my first bike in 1971, because the local shops couldn't be trusted to do it right. Now I don't need a book. Hint--if one of your ball bearings looks like a raisin it should be replaced before it does serious damage to the rest of the bearing.
Regarding the rear derailleur--refer to the attached BMP. There is a plated cable stop screwed around the right chainstay. When you remove the derailleur for cleaning and remove the shift cable for replacement and clean the spring housing (soak it in solvent), remove the steel cable stop that is attached to the stay. Clean that and reattach it over the scratch on the stay just forward of the dropout so the spring housing makes a nice elliptical loop between it and the derailleur. If you can't get the cable stop tight enough to stay put, see if your local shop has a smaller replacement. Now dry the housing and reassemble with a new shift cable that has a light coating of grease where it passes through the housing. Most newer bikes have brazed on cable stops.
It's highly unlikely that anything's seriously wrong with the rear derailleur that can't be fixed with cleaning and adjustment. It's all alloy and stainless steel. A bent B-screw or B-screw flange isn't a show stopper and the angle of the derailleur body looks fine as is. If the cage hangs straight down and the wheels spin, it's OK. I've seen jockey wheels running closer to the big cog than that, but if after adjustment it still runs rough, just get a freewheel with a slightly smaller cog.
This bike is a find. It's not exotic or state-of-the-art, even for 1976, but it exemplifies the best work in the traditional style of British builders of the period. It's interesting that it uses the Carlton-style wrap-over seat stay.