It has taken years, but finally Virgin is publishing its On Demand programming on the Virgin Media website. Hurrah!
Here's the URL for the HD programmes available online: http://www.virginmedia.com/tvradio/ondemand/new-tvlist.php?nocatchup=1&hdonly=1
If you click on a programme title it explains where you can find it in the maze of the EPG. Unfortunately, it doesn't tell you how long a programme is going to be available or what may be coming next, but it's a damn sight more efficient than scrolling through menus on the EPG trying to find the stuff. Appears to be fully searchable and sortable, too.
(Doesn't appear to include all of the Filmflex HD films though - just one or two.)