Worked out the sensor is faulty by borrowing a spare mirror (which I have to give back) so have found a replacement sensor on good old eBay (Main dealer said they weren't available separately.
Have managed to get the glass out but the next step appears to be to remove the outer plastic part of the mirror to then be able to replace the sensor.
Any suggestions as to how to remove it.
Will try and load some pictures but first time I've ever posted.
Cheers
Pictures in gallery

Where is the sensor? Mines a 63 plate if that's makes any difference.

Inside the nearside mirror, look at the gallery piccy I'm pointing at it.

All fixed, turns out it was a broken wire in the loom.