They should be able too, Ive never had a problem, but it depends on the tool behind the desk and knowledge.
When I've supplied the standard belt number they got a size up and I said roughly what length I want and they sent a few down to the size requested, then I send back the ones not up to size.
Trying to think what motorparts I used near bexleyheath, I'll try and find out of my mate Dan as he'll know.