Hi Nigel, my name is ***** ***** I will assist with your query this afternoon.
This is a fault within the digitst, the thermistor which reads the ambient temperature has failed hence giving an incorrect reading which is shown on the display.
Unfortunately these units are not economical to repair, the cheapest option is to replace the thermostat.
You can pick up a replacement for around £50 from Screw Fix and Wickes if your looking to get the job done this weekend.
Please let me know if I can be of any further assistance.
Yes, if you buy the same make and model, you can swap it out without touching any of the wiring as the back plate which terminates the wiring stays and you just clip a new one in its place.