Many thanks No Idea and Gavtech for responding. At least I know now that if I want dual recording, I have no choice but to take up VM's (Virgin Media's) V+ system.
My only concern is that once you have committed yourself to £150 for the box, you really have no control over VM's future pricing should they decide to introduce a monthly fee again. I know that is unlikely right now due to the Sky/VM conflict but who knows what will happen in the future.
PS I'm not normally pessimistic but I have been with Cable and Wireless/NTL/Virgin Media since its inception and nothing would surprise me!