I now have all the required ingredients to be fully Vagged up. Cable, Software, Windahs XP laptop and extension cable because the battery's knackered.
Can't get it working, though. The amusing little blue LED lights up on the cable, but when I do a USB port test on vag-com it shrugs its shoulders.
So I have either a hardware problem or a lack of skill. Or both.
I will stick with it and have a proper go this weekend. I lust after an extinguished airbag lamp.