My "usual suspects" would be Black on White (John Quantick) in Bristol, NewtonEliis in Liverpool, and ApertureUK in London - I was just looking at the latter and I think their Hasselblad specialists are now called "The Museum Shop".
As I recall, John Q has a fixed price for work on Hasselblad bodies, backs and lenses.
There was a guy who made himself known on here as an ex-Hasselblad engineer who would do services and whatnot, but he never answered any of my emails ...