I don't know anywhere in Chester, but you could try Silverstone Autosports in Garden City? - It's just off the end of Sealand Rd, but I don't know how good they are with a GTiR.
I would previously have recommended Dyno Demon on the Wirral because we had a couple of rolling road days there, but they seem to have dropped off the map.
I would previously have recommended Dyno Demon on the Wirral because we had a couple of rolling road days there, but they seem to have dropped off the map.