It's a while since I did it, but it went as follows:
1) undo the pulley securing nut so that it is still on but right at the end of the thread
2) clamp pulley in Workmate so that alternator hangs down underneath
3) apply small amounts of 3-in-1, WD40 or Plusgas to threads
4) make and deploy beverage of choice
5) put a bit of scrap timber over the nut and tap authoritatively with LR special tool No. 1 (hammer) until the alternator drops onto the nut.
Don't hit it very hard, as you can break the lip off the pulley and / or knacker the bearings, but it will go eventually.