I use Three Points. Unit 29-31 Bonville Business Centre, Bonville Road, Brislington, Bristol, BS4 5QR Tel. 0117 335 2333
Good value, courtesy cars available on request, or they can pick your car up and deliver it back to you. They've picked mine up in the morning and dropped it off where I work at the end of the day. Can't fault them.
Just picked up the car from BG Griffiths and they did a superb job. One broken coil spring and one corroded one replaced; a faulty injector replaced; 2 new brake hoses and new front suspension bushes. Plus they found a corroded area on the front sub-frame so they cut it out and welded in a new section for which they charged only £30. Total bill £660 and I think it's the best money I've ever spent on a car - drives like a new'un now.
EDIT: forgot to say that the total cost included £150 to fix the instrument pack which was missing lots of pixels.
Parkway in Fishponds - about as Northwest as you will get for a Merc Specialist.
Otherwise I recommend Golden Hill Garage on Brooklands Avenue (off Kellaway Avenue).
Very good guys who can sort 90% of problems and do very good and very observant servicing, i.e. they will pick stuff out and tell you very honestly if a part needs replacing - in fact they have picked up on stuff that main dealers have missed. Only thing they don't have is STAR.