[mattylondon]

I found this

Audi 2004-2006 A6/A8 all

Audi utilizes a sophisticated computer system to monitor tire pressure. If a low tire pressure warning light is observed, simply filling the tire(s) to specified pressures should shut the light off. If this does not shut off the light, then follow the "changing a wheel" initialization procedure.
Changing a wheel

When you change a wheel, the new tire must be initialized.

Press the "CAR" function button.

Select Systems in the CAR menu.

Select Tire pressure monitoring.

Select Initialize wheels.

When you change a tire, you must perform the Initialize wheels procedure. You must do this each time to change a tire. The procedure can take up to 20 minutes. During this time, the tire pressure and temperature will not be displayed because the sensors are being "broken in" and the learning process is taking place.

During the learning process the tire pressure monitoring system is only partially available It only warns when tire pressure drops below the minimum permissible specified pressure. This can indicate one or more tires as well as the spare tire. If this is the case, the warning symbol appears together with the message Tire pressure!

If this still does not fix the problem, contact your authorized Audi dealer for assistance.

And a useful PDF here

http://uberlame.com/a6_tsb/TPMS/TPMS%20Overview.pdf