Heatwell thermostat aube TH232 question

Hi all,

I have a few of these Heatwell Aube thermostats in a new property, for underfloor heating.

None of them seem to be actually controlling the heating in any way. Its just not firing at all. I've had them checked by an electrician and he's confirmed that there is power, which makes sense as the displays are on.

Interestingly, when I watch this video on controlling them,

there is mention of temperature sensors (the A/F icons) and when the heating comes on the little radiator icon appears.

I do not have either of those icons on mine, so I'm suspicious that something is wrong with them.

Does anyone have any experience with these devices?

Thanks
Shaun