2005 Cooper - poor performance

Hope somebody out there can help.
 
I've recently bought a second hand 2005 Cooper and it has developed poor performance.  It has had a new accelerator pedal and also a new lamba sensor as the engine management light was on.  New parts, still no better.
Symptoms are:-  Loads of exhaust when cold, feels like it is constantly being "choked" and holding back when cold and hot, although gets a little better once its warmed up.
Local dealer says it drives just like any other Cooper - which I don't believe.
 
I'm in the UK by the way.  Can anyone recommend a good Cooper dealer / engineer in the North West of the UK?
 
Thanks,

well, a compression test is always a good way to test your engine's condition. Just do a compression test first.