Craig-please read the CD posts on the "interior" forum.
You will see this subject has been talked about on a number of occaisions-mainly by me!.
The space under the rear floor is for a single cd-rom unit for the sat nav-it is DIN size so will not hold a normal CD changer (with the exception of possibly the Blaupunkt DIN 5 CD changer).
I have tried fitting my Panasonic 6 disc changer in the glovebox (to big) and in the boot (the false floor supports stop this, unless you lie it flat..)
Under the seat is an option, but not one I want to take.
My dealer wanted £600 to supply a changer, and the rear offside boot panel to house the changer (as per factory fit by Audi) and the associated fitting. Why is yours £400 and what are you getting?.
If that's the case I'm off to your dealer £400 for a supply and fit an Audi standard cd changer is cheap!.
Steve - 2003 1.4SE petrol, Silver, black/black interior