On the off-chance that someone from Virgin accidentally checks these forums, here's what happened.
On Wednesday evening after an hour on the phone I finally spoke to someone.
He insisted I go through the pointless routine of restarting my set-top box on the basis that some programs require a stronger signal than others and so might not appear in the menu until the system was reset! He didn't hesitate when telling me this, so presumably this nonsense is in the standard script.
After the fruitless reboot he put me on silent hold. I assume this was intended to make me give in and hang up, but I persevered and after 20 minutes he returned to tell me that he'd spoken to his colleagues and several other customers had already reported this problem. It was somehow the BBC's fault, and not Virgin's, but he guaranteed that the program would be available within 24 hours, and that I'd receive a phone call to confirm.
Needless to say, neither program nor phonecall materialised. The program had not appeared by the time the next episode was broadcast.so I missed it and will have to watch it on my laptop instead. Still no phone call either.