Finally finished the alternator on the BMW.. thanks to my Dad coming down so there were two of us, in the end the process went something like this:
Remove:
0) The engine top cover, air box and intake elbow
1) The radiator fan assembly (front upper slam panel off, upper radiator plastic cover off, lift the radiator fan assembly and fold in the tab on the right hand side then lift free)
2) The serpentine belt (T60 Torx into the tensioner on a long bar, take off the tension, remove belt)
3) Remove the idler pulley bolted to the alternator (lever off the front plastic cover, unbolt the T50 (or was it T45?) bolt in the centre with a breaker bar)
4) If you can, remove the idler pulley bolted to the belt tensioner (same as above. Sadly someone has previously mullered the bolt on mine by hammering an allen key into it

so we left it in position and levered the tensioner out of the way when necessary)
5) Get the car in the air on jack stands
6) Remove the front under tray (Thank you BMW for using half-turn quick release fixings!)
7) Remove the two large bolts securing the alternator to the engine

Remove the two 13mm bolts securing the PAS pump to the same bracket to give you more wiggle room
9) Unclip the main positive lead going to the alternator from the metal clips (one under the throttle body, one on the passenger head and one above the alternator)
10) Lever, wiggle, pull, curse, swear, remove knuckle skin, lever, curse, lever, crawl under the car, lever, get out, pull, crawl under, lever.. eventually you will be able to rotate the alternator slightly anticlockwise while simultaneously pulling it forward, down slightly and it will come off the mounts and out past the oil cooler thermostatic housing at the back.
11) Now put something heavy in front of the radiator fins (I used a thick car mat) and pull the alternator forward and tilt slightly vertical (drive pulley facing upward) till you can get to the 13mm battery +ve nut and the small clip-in connector at the rear. Undo these, you might have to get to the B+ nut from underneath
12) Withdraw alternator
13) Do the 30 second job of changing the regulator pack
14) ...
And as they say in Haynes, refitting is the reverse of removal. With more swearing. It's an absolute sod to get back into the space it came out from..
Hope I don't have to do that again in a hurry! At least the voltage now sits at ~14.3V rather than swinging from 11V to 18V..