Just a quick update - got a call from BMW GB this morning from a very polite guy who apologised profusely for the problems I'd been having and assured me they took such criticisms of their dealers very seriously and would be thoroughly investigating the matter. He'd already been in touch with Cooper who, he said, had no knowledge of the matter - strange given that I'd written to both the MD of the branch, and the MD of the group. ;-)
Now if I'd only had an apology from the branch rather than being told I didn't know what I was talking about, perhaps I'd still have been doing business with them...
While I'm not a believer in the 'customer's always right' idea, politeness and a willing to listen to a complaint never hurts.