Hi Clare
I live in Bristol, though tend to do all my own stuff, so no first hand experience of others.
If it was running ok before lay up, then I would have thought it would not be so much a tuning issue (as settings of carbs/Igniton would not change). It could be something like dirty fuel filter/ carb/s, deteriated carb diaphram etc.
There was an older guy working on classic cars at a quirky little garage in Frenchay villiage who I chatted to a bit back, he works part time now (haven't got his no to hand, though may be able to finf it if you want).
Also EH Motors, near Redland, apparantly work on classics.
Cheers
Dave