hi, mine had the intermittent misfire, usually when under load, numerous engine management lights on then off etc, (drove around the problems for months then decided to try the cheapest option first... ie coil packs.)
there is a pictorial guide showing how to replace them on this site, i just bought one coil, then tried it in each cyl till the misfire disappeared, cyl 2 in my case. cost approx 38 quid plus vat ea from dealers. (about a month later same symtoms... cyl 1 this time)
i havent changed any plugs or injectors, just the coils, mpg back around 37 and cars running good!
hope that helps...
remember these engines have had alot of bad press, but my belief is pretty much all the efficient dynamics petrol engines have suffered... just there is alot more of the 4 cyl engines sold, so they have got lumbered with the poor reputation, and poor resale accordingly.
if you want a trouble free car... dont buy a bm