4 companies come VERY highly recommended!

M A Developements - Mark Shead http://www.madevelopments.com/
Norris Motorsport - Karl Norris http://www.norrismotorsport.co.uk/
Motorsport Developments - Stu http://www.motorsport-developments.co.uk/
Supreme Car Services - Harvey Gibbs http://www.specialistcarservices.co.uk/

The choice is yours!