I have Thule aero bars I bought from someone who sold their A8 via this site and already have a pair of bike carriers which are the previous version of these ones:
http://www.skidrive.co.uk/cycle/prod...?partno=591000
They do work well on larger cars like ours - I originally bought them for my Seat Lean Cupra R as it wasn't approved for a tow bar and I didn't fancy trashing the bumper with pedal scuffs from a tailgate mounted one. You could see and feel the bikes moving around on that!
The site above is the cheapest I know of for Thule stuff in the UK