Hi Richard,
I live in Cambridge and have my Mk3 interceptor looked after by Autostore, from whom I originally bought the car 3 years ago,
https://www.autostore.co.uk" onclick="window.open(this.href);return false; who are not too far from you in Foxton, just North of Royston. The workshop manager there, Simon, can do just about anything on an interceptor and I have always found them very pleasant to deal with and very reasonable charges. They look after all my classic and modern cars.
Good luck finding a good Interceptor, they are fabulous cars and I absolutely love owning mine.
Cheers
Howie