Thermostat not switching off boiler

[BelfastGuy82]

Perhaps they have it wired, so that either stat can call for heating?

As I explained in post #6.

 

[Johntheo5]

Assuming two new stats, if you turn one off on its on/off switch as shown, and the other (the one you say isn't working normally) doesn't switch off the boiler off "normally" and you then switch that off also, see does it shut down the boiler. If it does, leave it switched off, turn on the other stat and turn it up to make the boiler fire up, then turn it back down, see does that one switch off the boilder without having to reduce it to 14C, if not switch it off as well, see if boiler stops firing.

You may not have the on/off switch, thats on the CM3S, also check that the switched Live out is connected to terminal 2.

 

[CountryFan]

I'm not in the know enough to understand what that really means, or how it relates to my problem...?

My apologies, just re-read and it's the stat misbehaving. I'm which case, get them back, could have been wired wrong