It was the front sensor. There are other ways of getting rid of the warniong light but none other that you can do whilst on the move. I was pretty relieved when I saw the damage. In my mind's eye I could see the cat in pieces - so I was lucky.
Cheers, Jim
PS I bought the lottery ticket. Not...
Here's a new one.. I was driving down the motorway when the engine warning light came on. This happens from time to time and it has been shown to be a false error. The way I get rid of it is to rev the engine up to the red line and hold it there for a while and the engine light goes back off...