I called up UFH and discussed the issue with my Devi Touch thermostat displaying an error message "overheated". Vicky kindly talked through possible issues as well as what an engineer callout with cover and what the next steps would be. Vicky also suggested some pre-checks I could do before an engineer came out and one of those checks would be checking the back plate and perhaps even swapping it with one of another room to see if that would still give the error message. Upon removing the backplate I noticed one of connectors had burnt out (see image) and melted the plastic and there was a noticeable burnt smell too. Clearly this was the issue. I swapped it out with one from another room and its working as expected again. I just need to purchase another devi touch thermostat to replace the damaged one. This saved me an engineer callout. Thank you Vicky for your help. 10 out of 10! This just shows that a simple phonecall can sometimes provide the help you need. Note : While I am not an electrician I was able to do this myself but would recommend using an experienced electrician when it comes to electrical wiring unless you're confident in doing this yourself.