Hi Richard, I live in Bath and work in Keynsham.
I first used the ex-main dealer Simonstone up the road in Brislington, who were ok, compared to some of the stories you hear on here but I couldn't say they were overly helpful and the price
I then used Automeo, one chap who works out of his house's back yard in Filton. A definite Alfa owner / enthusiast, but I have to say his style of service and communication was not really to my liking. I think he dd a good job, but there were always unexpected things on the bill he'd forgotten to tell me about and I got the impression that he perhaps didn't have all the diagnostic skills and tools needed for the modern alfas. However, I wouldn't not recommend him, I never had a problem with the work he did.
For the last 4 years of so, I've been using BenAlfa in Westbury - Alan Bennett 01373 86433. I can't recommend him highly enough. He too is an alfa enthusiast, old and new cars. Very thorough and reliable and reasonably priced. Very busy as a result though! Advantage of having EB Spares, alfa parts importer, just round the corner from him.
Admittedly westbury is about 30 min each way - car or train - from Keynsham, but he's only 5 or 10 min walk from the station and will probably give you a lift.
Not the closest, but I use him every time as from my experience he's dependable and keeps you in the picture with no BS or vagueness.
Finally... Alfanet just off the M32 in Bristol sell new & used alfas and maseratis, I seem to recall I contacted them years ago - not sure if they do garage work, but they certainly recommended someone in Bristol, but can't recall who - could be worth asking them if you don't fancy trip to westbury
Alfa Net: 95-97 Fishponds Road, Bristol 0117 952 0444