This temporary loss of channels has now happened twice.
1. Bring it out of standby and only the BBC HD mux channels are available. The EPG only shows the BBC HD mux channels. But ITV4 was being recorded at the time! Go to Recordings, choose the ITV4 recorded programme, choose Watch live ... and I can see the programme. The rest of the channels are now also available and all is back to normal.
2. I was watching a recorded programme which stopped. I can only see the BBC HD mux channels and the EPG shows only the BBC mux channels. This time a BBC1 and a ITV programme in the BBC HD mux are being recorded. I key in 10 from the remote control keys to go to ITV3, or 24 to go to ITV4, and the channels are not there. Try changing channels using the UP/DOWN arrows on the box itself and I only have the BBC HD mux channels. Cannot check further as I don't want to lose the recordings so will have to wait till they finish recording.
Very strange!