Via

My 2011 E87 had TPMS light come yesterday, checked tyres and one was slightly low (down ~8 psi) but now light wont go out again. From what I've read these don't have actual pressure sensors in tyres but work on wheel rotation. Is this correct? I have driven vehicle since pumping up tyre but light remained on, is there something I need to do to reset it? I'm bit nervous as car goes in for MOT work Monday and I go away Tuesday so don't want it failing on this now with no time to sort issue.