It's a standard Osprey in rather sorry condition.
Perhaps you don't yet have access to Sales although there probably have been some listed under Collectors, but an Osprey that really needs nothing doing seems to be worth around £120 so take it from there.
Yours needs a reblue (£60) (and with rust that heavy some pitting may not come out) and stock refinishing which will either be totally uneconomical if someone else is doing it, or will cost very little indeed if you or any buyer is.
I wouldn't really, but you could say I'd pay around £65-£75 for that. Sadly probably worth more as parts.
**WANTED**: WEBLEY PATRIOT MUZZLE END; Any Diana/Original mod.50 parts, especially OPEN SIGHTS