I ran some more tests and hr data is now being recorded in the gpx file. I can see the data recorded as such:
<extensions>
<osmand:speed>1.8</osmand:speed>
<osmand:hr>98</osmand:hr>
</extensions>
(Not sure what is now different because I didn't change anything!).
BUT, the hr data is not being read/recognised when I manually import the gpx file in to Strava?
I know this is nothing to do with OSMand, but Strava is the elephant in the room so-to-speak and where millions of us centralise our activities, so it would be good if Strava did recognise external data from OSMand either via manual import (using tcx format?) or -preferably- a direct share/sync link with the OSMand app to Strava.
Many thanks