Managed to do this!
I removed both sensors without taking out the glass, or removing the back off the mirror pod. Fiddly, but you can ping the sensor out of its locating hole, feed the wires out near the hinge to expose some mini spade connectors - allowing me to swap the sensors over.
Using a sucker on the glass gives good control with no risk of cracking.
And using a springhook
Note the spare I had in the drivers mirror was slightly different, and I had to enlarge the locating hole in the passenger side to accommodate it. (I guess maybe the mirror has been replaced at some time, maybe from a P1?)
I was lucky that I had a spare, but a similar process could be used to expose the spade connectors if you were wanting to replace the sensor with one bought from Maplins or RS Components as some other threads indicate are available.
checked with FES and outside temp now reads correct ! (22 Degrees ...swoon)
Failure of this sensor should not be underestimated
, in my case because it was telling the aircon ECU it was -16 degrees outside the car, as a result it was making the heater blow waaaaaaaaay to hot regardless of the dial setting, and was permanently stopping the aircon from turning on the compressor AT ALL (I've read this is to avoid risk of freezing it up if it thinks it is too cold outside?? seems plausible) These symptoms can be overcome by disconnecting the sensor at the mirror root plug, but that is only a workaround, fixing or replacing the sensor is the way to go IMHO.
thanks PKR and others for help, Sean