General Autos (Abbey Street) is very good. Raj is excellent, but he seems to be extremely busy these days.
VW Enterprises (Empress Street, just off of Walworth Road) specialise in VW and Audi (VAG). What the guy there (Darren, I think his name is) doesn't know about VWs and Audis can be written on the back of a postage stamp. He recently sorted out a similar issue with the timing chain on my Touran and also sorted out sluggish acceleration at the same time. Thee guy is better than a main dealer. Very highly recommended. No affiliation etc. Phone number: 020 7703 8689.