So after searching on here for quite sometime everyone has asked the same question about a million times! Normally regarding the S3 Bumper and the S-Line rear valance/diffuser.
In my case I have a 2010 2.0TDI 140 Black Edition which has this rear bumper:
I want this rear bumper / valance: (from the 170 black edition)
The rear valance pops out and is interchangeable and the part number from Audi is: 8P0807434F1RR and the price is £90.54 for the part. Fitting is simple, 2 bolts on the back and pop out and then replace with the new valance. Of course then a trip to your exhaust centre for a twin exhaust tip / backbox and hey presto much sportier looking rear end.