Hey, have you seen this site?
http://www.unixnerd.demon.co.uk/garages_seng.html
it seems to be more about older cars, but still gives an overall idea...
In my experience, Altwood Taplow (now Stratstone), good at sales, not so good at service; HR Owen Chiswick (on the Great West Road in Brentford) - had to use them for some warantee work on the e39, and they just didn't want to know - basically get the job done and get rid of me. Very poor service, so bad, I ended up in Wood in Odiham
Actually got very good Sales service (the Business Manager, Noel, is excellent), although their initial p/x offer wasn't the best deal. Haven't done enough miles yet to see if their servicing is up to scratch, though.