Update to the above. I rebooted the box again and all channels came back. But here is the strange thing, the BT UHD channel - 555 - no longer appears in the channel guide so they must have done something to remove it (after my second call) which only became apparent after the latest reboot. Now the channel skips directly from Freesport 553 to the news 601.
The big question is do I phone virgin, wait the usual 15 minutes or so to get connected and cancel the engineer or do I do nothing as punishment for ruining my nights sports viewing and not seeming to know or at least tell me that the cause of the problem was apparently BT sport's UHD?