My rear os tyre is slightly under pressure but the readout on Carista shows the speed of that wheel to be slightly slower than the others!  Is that correct  - my brain tells me it should be faster?  Am I being stupid🤣

 

With inderect TPMS you are correct, Assuming everything else is constant the wheel with low pressure should rotate faster; how much faster it needs to go to trigger a warning, I don't know. My Karoq has never given me a warning but previous other cars have and there was significant pressure loss, about 7-8 psi before I got a warning. The difference could be different size tyres, different amounts of wear, different recommended pressure front and rear, not being on straight road, etc.

So it thinks the circumference is less than what it was logged at when the tyres were at whatever pressure and the TPMS reset. 

It can be happy at what ever they are at and it is just looking for the change in how many rotations of a wheel and some are sensitive and none not.

 

It is not always a loss of pressure that can set off the TPMS warning, a binding brake or overheating bearing and a wheel getting hot and a tyre expending can set it off.