Update:
After a factoty reset, re-downloading the Element App and logging out and into everything (plus RWGPS) and that not appearing to work, I was about to mail Wahoo Help desk.
One last fiddle saw me update the Wahoo via itself on Short Power press/Settings/System Info/Check for updates and apply them rather than my usual acceptance of updates when I fire up the Wahoo.
Happily, that seems to have sorted out the issue!