http://burghfieldstarterandalternatorcentre.co.ukThese guys were pretty good for the m3 and I think mike used them for his st205.
There was an old school guy in north camp I used to use who was fantastic but has now shut down (I assume old age hottie better of the old fella in the end as he must have been 65 20 years ago!
There are many reports on the m3 forum of the new/replacement Chinese alternators failing within warranty period and it's just not worth the hassle IMHO. Most of the uk repair places seem to use decent branded components and are cheaper!