Ever since putting the girls in tubs for separation purposes I've found myself adjusting the temps almost every two days although everything was running well at that.
I got a scare this morning when I saw that the hotspot had hit 41C . Thankfully I observed sensible behaviour displayed by my gravid girls. Suzy was simply cruising about while Fawn had the majority of her body stretched towards the cool end with her head in the 'danger zone'.
The tubs are on a mat that is statted directly (ie sensor in direct contact with the mat.
A quick investigation has led me to deduce that the tubs were causing thermal blocking as the mat was hotter to the touch where the tubs were - whereas the free areas on the mat were at the perfect temp.
I tried two options:
1) creating an increased air gap between tub and mat
2) covering the sensor with cardboard to simulate the correct temperature experience when covered.
Option two has produced the best result. Temps are steady once again.
Fingers crossed no damage has been done and the girls did their job of thermoregulating as I observed.

Note - temps were always steady before but the majority of the tubs were empty and the occupied enclosure were spaced quite far apart. Having fully loaded tubs appears to have caused this issue, as I do like a thick (1 inch+) layer of substrate.