Thanks for the suggestions everyone. In the end I managed to combine two non-working Cinestills into one working one. I had one that had a good touchscreen but a non-working thermocouple relay and one that had a good relay and a non-working touchscreen, so I swapped the touchscreen over. I think the fault was a bad solder joint in the ribbon cable connector on the touchscreen. For anyone who finds this thread and wants to do the same, take a sharp narrow blade and insert between the screen and the silver ring at the 3, 6, 9 or 12 O'clock position and then prise up the thin plastic face cover. The reason for these positions is that there are screws underneath here, so a small screw hole that makes it easier to slide the blade in at a angle and prise up. There is a lot of adhesive under the plastic face cover so go carefully. Once this cover is off, you will see four screws that hold the silver surrounds ring. Remove these and then the ring. Under this there are six more screws that hold the display/touch button unit in place. Remove these and the display can be pulled out. There is a thick ribbon cable behind that connects this to the unit, which can be gently prised out. It will only insert one way, so no need to mark it.
I don't find the Cinestills particularly accurate; when I calibrate with my Kodak process thermometer they tend to over-read by a degree or more, I guess this is an issue with having the thermocouple right next to the heating element. To get a consistent 38 degrees C in my C-41 water bath I generally set them at 40 degrees, though to be fair, this is a sample size of two.