There's quite a lot of work involved in replacing the sensor
Time to be honest here: I'm not much of a nuts'n'bolts type of person. However, when I attended my bike refresher dates this weekend I was given the details of a bike doctor. I have sent him the photos and the advice from this forum and asked if he'll carry out a full service (brakes, gears, etc) and also replace the sensor. He said he's happy to try and fix her up.
eBikeDirect were great too. After verifying the model year (2015) they said they'd pop a free manual for the bike into the package along with the sensor.
Hopefully I'll be back in the saddle by the weekend (fingers X'd).
I'll update with details once I'm all done.
All of your kind advice is really appreciated.