I've been taking my 2005 GTI to VAGtech in Bletchley for about 5 years.
Their customer service is second to none, and prices always seem very reasonable.
They're fully set up with all VAG equipment and ex-VAG technicians.
Always very helpful, and they remember returning customers and their cars.
Only drawback is that their workshop has no letterbox, so can only drop off car when they're open, and it's a long walk from Bletcheley station if you've forgotten to organise a cab!
My car is back there for a service on Monday