I've had a repair to my Mamiya 7 made by Sendean, St Anne's Court, off Wardour Street in Soho. It cost notably less than sending it to Mamiya's UK agent and they can access parts from Mamiya.
Sendean Cameras claim to be "London's leading camera repair store". They are located close to Oxford St. and quote £75 for an "average" repair on a Mamiya back and £80 for a lens.
Camera Clinic who interestingly also claim to be "London's Number 1 camera repairers" and are similarly located.
Perhaps a quote from each might determine which conforms with your affordability hurdle