FL S8 won't start

[S8yot]

I seem to have the same problem Mick had on his pre FL. My 2001 S8 runs (corretion, ran!) fine but on a couple of previous occasions hasn't wanted to start. No difference hot or cold. Engine management light hasn't come on at all driving and immobiliser light goes off with ignition. It turns over but not the slightest sign of life. Car's at home at the moment (relucantly towed about three miles at less than 30mph). AA guy checked and no spark. He mentioned unprompted that he thought the likely cause is the sensor.
I don't mind risking the £100 on a new one if I can fit it myself as it will avoid the cost of getting the car delivered to the garage. I can't find anything on the interent about it and I would appreciate some pointers on fitting. How accessible is it? Is there an easy check I can do with a basic multimeter to confirm it is the culprit? (DIY skills are fairly basic!)
I'll be really grateful for hints & tips.

Mike