As suggested, the thermostat required a look. I decided to take the easy route and turn the breaker off and switch the thermostat from the 'problem' bedroom heaters with the functional thermostat in the ensuite.
The problem with the constant ON moved from the bedroom heaters to the ensuite heater so I guess it's pretty clear that the culprit is the thermostat.
Now it's off to the store to replace the Broan/Nortron 1T22 thermostat.
Thanks for all the help!
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