I've been told I need a wheel sensor which will stop the BAS/ESP light coming on, how much are they and how easy/difficult are they to fit? Are they a main dealer part too?
Try disconnecting the battery for a few minutes then reconnect it. Start the engine and turn the steering wheel lock to lock 3 times. This is the normal method for re-setting the light. If there is still a problem then a least you have removed one possible cause for the light being on.
Thanks for your advice, I'll give it a try in the daylight, I replaced the switch on monday and had the car plugged in, sometimes the light comes on as I turn the key, other times I'm driving for anything from 2 to 30 minutes and then it comes on.