I have a Warmup 3iE thermostat using an NTC 10k "probe" (which is what they call the thermocouple embedded in the floor, under tiles) in a very small bathroom. It's the only heat in there and so is pretty important.
It's been working well for nine years but now has had a Mad Hatter hissy fit! The floor is cold but the unit is reading 50degC!
So, I checked the resistance of the mat (the actual heating element in the floor) to be 87.6

and the probe to be 13.5k

when the floor was really at 18.6degC (measured with a thermal camera).
I have been told conflicting things:

1. Since the probe is reading about right (data tables for the NTC 10K show that this resistance is spot on for 18.6degC) it must be the actual controller that has gone mad and I need a new one at around £100.
2. Regardless of the probe resistance, if you set the controller to measure air rather than floor temp and it shows about right (which it does) then it's the probe that is mad not the controller and I need a new one at around £2,000 (smash up the old tiles and heating element, lay new element and probe, lay new expensive tiles chosen by the boss).

Can anyone please help with guidance/advice - I will beg if necessary.