TVR recommend that their cars be serviced every 6,000 miles, with a full service every 12,000 miles. A detailed TVR service schedule can be found here.

Authorised TVR Service Centres

If your S has a full TVR service history it's probably worth maintaining this by getting the car serviced at an authorised TVR dealer. However, dealer servicing does tend to be expensive. Also, not every TVR dealer will service the S series of cars, as (with the exception of the V8S) they're regarded as old models, use the Ford powerplant rather than the more common TVR or Rover V8s. 

A full list of authorised dealers is available from TVR's web site, http://www.tvr.co.uk

Unauthorised Service Centres

A cheaper option than the authorised TVR services centres is to find a reputable garage. It is important, however, that they have a good knowledge of TVRs in general and in particular the S series, and can locate parts with ease. All TVRs, especially the early models, borrow heavily from the parts bins of other motor manufacturers, and TVR don't tend to stock the majority of parts for the S series. 

There are quite a few very well regarded S-savvy garages dotted around the country. Usually it's best to check out the forums on www.pistonheads.com to see people's recommendation. However, check out the contacts list.