Thankyou for your helpful replies ! Eventually I realised that a rear tyre valve was losing pressure very very slowly causing the sensor to activate. Once I got the valve changed the problem with the tyre sensor disappeared completely and the sensors have not activated for months now. If the sensor is triggered after a long run - when the tyre pressures have been reset earlier - I just check them for any obvious puncture/air loss then if all ok I just cancel the sensor message . But I only do this once . Fortunately the sensors are fine now .