Not quite sure I have it sorted yet, but, thank you all for the suggestions and discussion, I have learned some things. Pretty sure it’s a communication issue. I looked at the browser version of strava today and they have a readout page there I didn’t know existed ( I only ever use iOS). In the readout it shows the sensor works for a few minutes and then cuts out. So either a BT communication issue (I’ll try move things closer) or a sensor gone haywire.