Well, I suppose I shouldn't advertise,but you asked for the details Steve, so here they are:
Pioneer Tuning Specialists, 6 Madeley Street, HULL HU3 2AH Tel 01482 227848. I've never been there, so I can't say personally whether they are any good, or good value, but they are apparantly a couple of ex-BMW-dealership guys who know their beamer stuff and I've heard they're ok.
If it is simple mechanics of fitting a part, I understand that Monsieur Auriol of Auriol Motors, Uni1, Spacemade Park (where???), Stoneferry Road, Hull HU8 8DA Tel 01482 588595 should be able to do this business. His first passion is French cars, but he knows his German cars too, so I'm told by a fella in the BMW Driver's Car Club with a 535, who takes his car there for general work and would recommend him as doing good work at a good price.
Take your pick! (And let me know if you would recommend them to me!)