Their are many tuners who can build you a decent engine.

MAD - M.A. Developments - Mark Shead
SCS - Specialist Car Services - Harvey Gibbs
SCC - St Albans Car Clinic
MSD - Motor Sport Developments - Stu and Kenny
Norris Motorsport - Karl Norris
Can't think of anymore @ 1:35am

Alot of people on here favour their own tuner for various reasons, but just because they had good service and a good engine, don't mean you will. Phone everyone up above and see who helps you/offers you the best advice and products.

Put a post up here (DO NOT MENTION NAMES) and everyone will give you honest advice on anything you need to know.