Thanks for the advice, I'll remember that next time it happens.
Strangely, I can't seem to get (or get to, perhaps) any on the TV's built-in 'Freeview' channels now via the 'digital' input; not that I need them, I watch TV through the Virgin box or via the PC, but I still don't understand what's happened to them. I used to get them if I pressed A/D, but not any more