By way of a final word on this topic, I bought a cheap (ish) Autel Ap200 OBD2 reader, paired with the Autel App on my phone. This interrogated the TPMS system, showed nothing untoward other than a fault code, but all sensors were showing life. I cleared the trouble code and so far it has not returned. Fingers crossed. Thank you to all who offered words of wisdom - much appreciated.