Just switched on to be confronted by this:
Menu/Media/Guide made no difference to the video but displayed the correct thing on the front-panel display. Changing channels cured it. Never seen anything like it before...
The item was about a pub sign featuring a Suffolk Punch. The video corruption was not an overlay - it looked like it was double amplitude or similar.
I tried turning the TV off/on again and selecting another HDMI input, but the fault was definitely output related on the Humax.