I own, shoot and rebuild Original/Diana Mod 75's and luckily you aren't that far from me, I'm in Swindon.
I'm airgunsmith to The Airgun Clinic but also take on work privately. I use my mod 75 every week in the Swindon 7yds League (paper version of bell target).
I keep a small stock of parts for these guns as well as servicing them so gimme a pm and we can sort out what you need.
medal, medal, medal...