As you are in sunderland i can give praise to a garage in Boldon Colliery (Robertshaw Racing) although its newcastle, i have a mate who works there called Steve, hes a member on hear and has a heavily modded Cooper S. Hes done work on my car and hes a Top mechanic. Here's a link:
http://www.robertshawracing.co.uk/
They also have cars in the British Touring Car Championships and MINI Challenge so thats gotta be a mark of how good they are. Ohh and they will be chaper than BMW
Maybe source the clutch yourself then give them a ring for a price on fitting it.
Scott