Foreign Car Hospital has been a Hyde Park car repair institution for a number of years. They are a small independent mechanic outfit that takes CASH only. You're probably thinking this is a bit dodgy, but everyone I know recommends FCH because they are honest and reliable. The guy who runs the place is knowledgeable and you should consult him by phone before taking your car there. FCH does mostly mechanical work, so if you need your brakes done, fluids changed, parts repaired, etc, it's a good place to go to. Generally, leave your car in the morning at the shop and pick it up in the evening.
FCH's prices are about the same as the local repair chains. However, I - and others - feel that the owner and mechanics are more reliable than the chains.