Next move was to get the one off the bike - fairly simple, took longer to get the plastics and footpeg hanger off really. It's fairly tight, but came out easily enough. The pinion was absolutely caked in crap. Then I noticed it was a Bosch, not a Valeo. Hmmmm. Anyway, the new one landed next day, and on comparing the two, it was obvious that the Valeo was around an inch longer than the Bosch, and a fair bit heavier too. Googling revealed that BMW only used the Bosch unit for a short time before upgrading to the Valeo, which gives around 20% more oomph when the button's pressed. I had to remove part of the gearchange linkage to get the new one in, good excuse to give it a clean and lube while it was off, then it took ten mins to fit the Valeo.
The moment of truth! I pressed the button.....wow, it nearly turned the bike off the stand, such was the difference! So, a reasonably priced, easy-to-do job that's made a real improvement to my bike - I'm a happy bunny!

Pete