Hi as you said its probely one of the sensors the best thing to do it go to a good independent garage and have them read the fault for you and this will then let you know which one it is and then you can just go ahead and change it job done!!, i hope this helps Donald.
Thanks for the advice; your reply arrived after I'd set out to have a look!
I took the NS front wheel off first just to have a look. With it off I found the wire from the brake pad wear sensor snapped. I know this is unrelated but why haven't I got a brake warning light up?
Anyway I couldn't remove the sensor, 4.5mm allen key and I only have 4 and 5 in my set, so just cleaned up the toothed wheel and checked the sensor connector - seemed OK.
Whilst under there it occurred to me that at the MOT, 6 days ago, they had reported having to resecure the OS drive shaft gaiter. I whipped that wheel off and what did I find? Grease all over the toothed wheel! Closer inspection showed that the gaiter clip had come off and was in fact now on the toothed wheel stopping the sensor from seeing any rotation!
Cleaned it all up, regreased, resecured the clip, and so far so good. It would seem that this clip hasn't damaged the sensor - phew!