Newbie needs help!

Hi,

Bought a 2004 A4 Estate 1.9, the airbag light has been on since a while, any ideas on what could be wrong and if I could sort it myself? If I need to take it to a garage then appreciate if someone could recommend a decent one in London, I live in South West London.

Also, the mpg indicator seems quite erratic, keep fluctuating from very highs to lows to no readings while driving, is this normal? 

Seems the previous owner had once pumped in Petrol instead of Diesel not sure of the damage, said wasn't any, but could this be the reason for the erratic mpg display?

Sorry if I sound a bit silly but this is my first Audi and so somewhat apprehensive, any help would be much appreciated.

Thanks.

Farhan