Back to reality.
Have been looking at the router stats for connections and disconnections from the Humax boxes.
HDR1000S connects every 6-21 minutes for about 8-12 mins
HB1000S connects exactly every 25 mins for 7-11 mins.
Both boxes are registered with the Freesat App but the HDR1000S is the default.
So it appears that the app must actually be unpaired from the box (and not just set to non default) to prevent ET calling home at alarmingly frequent intervals.