Autokare on Ditton Walk have been recommended to me frequently. I'd phone them up and tell them what seems to be wrong, perhaps they can tell you whether to get a full service or just something replaced. I've also used ATS on Histon Road for my MoT and was very pleased with them; less so with the repair I got done at Autokare, where they actually didn't look at the thing I told them was wrong (pretty serious; oil leak) until I turned up to pick the "fixed" car up. D'oh. There's also a little garage quite near Newmarket Road Tescos which my breakdown company took me to, I can try and ferret out their details.