I was there yesterday getting a cambelt change on our Sportback 2.0TDI. Have to say, the place is absolutely astonishing.
It's not just five or six floors above ground - there's TWO basement levels! One is used for secure storage of new stock and the lower basement houses the servicing bays. There's a café, loads of display models littered everywhere and also one floor given over to something called "The Quattro Room". This is used for corporate conferencing (apparently, BMW have used it already) and is home to a £100k TV and vintage Audi cars, including Alan McNish's winning R8.
The service was excellent too. A driver dropped me to my office in Chiswick and en-route explained that the Highways Agency have had to tell the dealership to lower the blinds on the upper windows at night, because it causes traffic jams on the elevated section of the M4!
He added they get a lot of "tourists" at the moment but it is well worth a visit. The service advisor said he'd show me round next time!