Well I changed my aerial and now got all the MUX's back at full strength.
I think my old aerial must have not been a full wideband, and was only barely able to pick up channel 67, even thought my TV had them with a low or disappearing signal.
Funny I'd not noticed it earlier, but as a Sky user I only use Freeview/BT Vision as a backup should I wish to record more than two programmes at any one time.