SCARTs are the invention of the devil (well the French anyway, which may or may not be the same thing

). They have this very annoying habit of trying to part company with whatever it is they are plugged into.
So the first thing I do if there is a SCART lead involved is give the plugs a firm push to ensure they are properly engaged with the socket. I have known SCARTs that partially unplug so you lose video but keep audio (or vice versa).
Also worth trying a different SCART lead if you have one, just to eliminate any issues with the lead. Also if the TV has more than one SCART socket try it on one of the other sockets. Always possible there is a dodgy connection in the SCART socket you are using. Equally if the Virgin box has TV and VCR SCARTs on the back try swapping from TV to VCR SCART to eliminate any problem with the Virgin box. NB the VCR SCART probably won't do RGB out like the TV SCART so you may have to change video modes on the telly if you are using RGB at the moment.