According to Humax this can occur when the unit is in the vicinity of a large screen TV.
Wasn't that something to do with the sensitivity of the front panel getting interference from the TV? In fact on their Facebook page:
"If you are having random changes,
- If you can get access to the front panel option, please change from Very High (Default) down to High -> Medium -> Low and Very Low and select the level that suits your environment.
- If you are not able to reach front panel option before it switches off, temporarily move it from current location and change the level.
*If Very Low doesn't solve the problem, please check your HDMI CEC Link between HUMAX and your TV"