In the mythfilldatabase log file the error message was
(Run) - tv_grab_uk_rt --capabilities failed or we timed out waiting. You may need to upgrade your xmltv graber(Run) - Grabbing XMLTV data using tv_grab_uk_rt is not supported. You may need to upgrade to the latest version of XMLTV.
- tv_grab_uk_rt is a python script that grabs the the UK Radio Times XMLTV listings
- mythfilldatabase runs the above (expecting it to be on the path)
- mythfilldatabase is run from mythbackend
- mythbackend is run as a systemd service (without the python folder on the path)
Which adds the Python path to the environment for mythbackend. Now tv_grab_uk_rt works fine.