What connections are on the virgin cable box? I doubt it has a tv aerial cable these days, more likely has HDMI and may have SCART. Your HD recorder may not have any form of aux input... it may only have aerial (VHF/UHF).
Something else to consider... the virgin cable box may have encoded the output, using something like Macrovision.
Also the virgin cable box may be showing HD content, via HDMI connector but it won't show that via SCART. So if you are after quality, you may not find you can record the HD signal.
I have a virgin cable box and a freeview hd recorder. the hd box's signal comes in from a standard aerial which is shite. I can't get all channels at the same time. Is there a way I can feed the signal from the virgin box into the hd box so I can record easily?