I have just measured the spring with a cheap Aldi vernier gauge so I'm not 100% sure on accuracy but here goes. The length is 220mm, the ID is 14mm, the OD is 20.1mm, the coil diameter is 3.05mm and there are 28 coils and there's a 3mm spacer fitted too. Does this sound about right for a 1965 HW35?
Even though I can't detect any air coming from the breech I think a new seal would be a wise buy. Are these a standard HW breech seal?