Hi mate, Theres two I would go to.
1) Middlesex tuning, in South Harrow.
2) Scheldt & Pettit, Fiat specialist. Northolt Road, South Harrow.
Both are very good, Middx tuning has alot of sophisticated testing equipment and are an authorised bosch dealer.
S&p are a couple of old chaps who know thier cars inside out, and do alot of restoration work.
Hope this helps.
Regards