UK is closer than US!

Mamiya agents are Johnsons Photopia:

http://www.johnsons-photopia.co.uk/contact.htm

My favorite independent repairers (although I know several good ones) are Newton Ellis & Co in Liverpool:

http://www.camerashop.btinternet.co.uk/

Hope this is some help!

Regards (or as you're in Stuttgart - Gr Gottle!),

David

PS: In my previous reply, I of course meant focusing ring/mechanism instead of aperture ring/mechanism. The repairman recommendations still apply!