Honeywell problem baffled plumber

[DP]

Something strange happened last night, the heating came on and stayed on until it got to 22. We tried lowering the setting but it didn't click off. Anyone got any ideas?

Yes, my suggestion is call a heating engineer who is able to diagnose with tools and test gear to get to bottom of the issue. No saying this is the case with every plumber/ installer/ electrician, it is often the case that a boiler installer is in rocket science territory when anything electrical is concerned and electrician is same when boiler internals are concerned. Recently had to replace a PCB where the spark had connected 240 volts to low voltage demand terminals