A useful response, thanks John.
I'm booked in for a battery change, anyway, as S/S hasn't been working for the last 8 months. I hadn't really thought about it, just putting it down to me not 'activating' it properly. But if that doesn't solve things, then It'll be on to the wheel sensor next. Would be a real pain if that, and the TPMS, went at the same time. I don't have a reversing camera, only the audible sensor, which still works. That may also indicate it's not just a battery issue.
I suspect I've got a duff battery, as well as some failed sensor.