For Mamiya cameras & lenses of all kinds, I'd recommend:

Chen Parmar
020 8737 7936

He's located out in Hounslow, West London. I was referred to him by Fixation of London; as they work nearly exclusively on Canon & Nikon gear, they told me they send the occasional bit of Mamiya kit that comes their way to him.

I've hand-delivered my gear to Mr. Parmar on the occasions I've used his service. He always has everything ready within four or five days, and his prices are fairly reasonable. He's fixed TLR lenses for me, serviced an RB-67 lens, and repaired a back that had broken down.

I've been completely pleased with his service.