Steve,
I can very strongly personally recommend a chap called Stan Lawrence of Surrey Amps (google it). He's just done some work on my RKII head and gave a fantastic service, well priced and lovely bloke into the bargain. If that's too far to travel suggest you call Oresta at Westside (again really helpful chap), the UK distributors of Mesa and he'll put you in touch with someone more local.
Regards,
Piers