I think @grahamh can help you here. I don’t think you need to start from scratch you just can reset the user file. But I am not anymore sure if it is in /etc/tvheadend or in /home/osmc/.hts/tvheadend/
sudo systemctl stop tvheadend
rm -r /home/osmc/.hts
sudo systemctl start tvheadend
or for a deep clean, uninstall from the AppStore
rm -r /home/osmc/.hts
reinstall
Finally got it working so thanks to all who responded. Unsure exactly how it resolved and sometimes it seemed that a restart made no change at all but then another one did. So fingers crossed and a much better system than my existing Pi B+.