Thanks alot for all the advice. The dimming thermostat is actually linked to a light bulb, not a heat mat, I appreciate this works similarly anyway.
At the moment the sensor is right by the basking spot/rock and is obviously getting heated up too much. I'll do a little trial and error, and move it along so it's further away from the basking light, probably into the middle of the vivarium. At the moment both the warm end and the cold end of the viv are too cold so obviously a little playing about is in order.
Added question: what temp should I set the thermostat to for a beardie?