Lack of power - Cheap Mechanic required

My car is an Audi A4 Avant TDI ( W reg 2000 ) .
It has developed the loss of power that I have read about in several previous threads . Use to be fairly pokey , now struggles . I was wondering if anyone knows of a decent mechanic who knows how to cure this fault with out charging a small fortune. I am down South. Between Portsmouth & Guildford on the A3 motorway.
Alternatively has anyone used VAG for this and if so what do they charge.
Anybodys help would be greatly appreciated.