Hexham HC, R&P Woods? in Heaton, Newcastle. Astonparts Blackhill, Consett. Falcon in the boro. SSCC in Boroughbridge. I'm sure there are others but people don't appear to be very forthcoming with suggestions
For S6 engine cars I'd still consider one of the ex TVR dealers. For a Rover or Serp V8 you're better off finding a local reliable mechanic for mechanical type problems.