It's a BT problem, as tried the aerial direct into the HD TV and all HD channels are fine direct on Freeview.
Tried several retunes of the BT box to no avail. No sign of them at all.
Possibly too much signal.
What is the signal level and quality showing?
If it 100% signal, then its most likely the YouView box is being overloaded. That will cause loss of channels.
A signal reduction can be made using a variable attenuator like this.
On one of the non HD channels (Channel 4) I can get the signal strength is 94% and the signal quality is 100%
The signal level should be fine, however its possible that the transmitter power has been increased which may be the issue. I would try checking a few other channels, as it only takes one of the muxes to be 100% +, to overload the box and prevent decoding of the signal.
It could be an issue with the YouView box, but I would expect that it would affect all channels, as its the same internal decoder.
You could try powering it off, then on again, to see if it helps.
I'll try that thanks. Tried few other channels and they're all ok. Just the HD ones totally disappeared from guide etc.
If it doesn't resolve will just switch aerial to Freeview receiver in TV and then use Youview box when watching BT sport. No great loss- Youview does seem quite unreliable.