Mamiya Repair In London
There are indeed several good places in the UK to get Mamiya TLR repairs done, as others have mentioned. If you're in London, and don't mind a trip out to the wilds of Hounslow Central, I can recommend Chen Parmar. (You can post your lenses/gear to him if you don't fancy an hour on the Piccadilly Line.)
I found out about him when the guys at Fixation of London (the most prominent Nikon/Canon repair establishment here) told me they sent Mamiya stuff that came their way to him for service. He's serviced both TLR and RB67 lenses for me.
He's quite good, pretty rapid in turnaround, and very reasonable on price. You can reach him on 0208 737 7936.