Thanks Tell - the upgrade is popular amongst the Audi A3/A4 owners, and it looks simple enough, with the adapters for wiring available readily, perhaps the only complexity is the vag-com programming so that the nav instructions appear on the "hi-line" mfd in the instruments.
A user of the audi navigation plus who had chance to drive a VW with the same nav system but with the different screen and controls said the VW system was better to use.
I love my tom-tom however the costs for the XL Altea Seat dashboard mount, plus the costs to sell my 700 and buy a 710 to fit the adapter all add up, and still leaves with the hassle of security for the tom-tom. I'd much rather loose a little in functionality if the ease of use and security aspect was better.