Would need a lot of work and money to bring it back into rideable condition. Check the seat post and stem aren’t stuck otherwise it could mean mission impossible. Assuming of course you want to restore it.
Well, it's a Worksop style frame number but it will have actually been made in Nottingham, in the lighweight facility rather than on the mass market gas pipe frame production line.
If you look carefully at where the rear mudguard is attached, there appears to be a Reynolds tubing sticker.
I agree it is likely to be a 531 frame as 501 only appeared in 1983. It's potentially a good ride if a bit of effort is put into getting the rust off and giving it a coat of paint. Neglected 531 bikes make ideal low-profile hacks if given a mediocre looking paint job with no decals applied after. They ride well but don't look attractive to thieves.