[SOLVED]
Heating engineer came this morning. Thankfully as he arrived, the issue was happening (heating was stuck on full blast in bedroom zone even though temperate had been met).
After my explanation of events, he performed troubleshooting, including:
Testing the wiring to the thermostats...
Thanks @stem & @CBW
You are totally right @stem, I naively hoped replacing the stats would be enough.
Really appreciate the time you took to read and reply to my issue - I will call a heating engineer first thing tomorrow and follow up with the result.
Thanks again
Central heating
1 combi boiler in the kitchen IDEAL Logic Combi ESP1 35
1 thermostat in hall Danfoss TP5000Si
1 thermostat in upstairs bedroom Danfoss TP5000Si
2 motorised valves in upstairs airing cupboard Danfoss TYPE HPA2
Heating is not turned off
Both zones - but only 1 zone at a time -...