2005 1.6 A3 Sport.

Two days ago i was driving with a bit of a heavy right foot and the light popped on. Ive read its probably down to the intake manifold or the Lambda sensor. Since the light has come on my MPG has dropped from around 35 to low 20's. Is there anyone in the local area (Kings Cross) that can assist and possibily give the car a scan or help advise on what to do next!?