I can't see the restriction on wheeled bags anywhere on the Virgin website. Anyone got a pointer?
It's no longer the case. I believe the restriction was lifted in 2004.
There was no such restriction when I was working for VS. As long as the bag fitted the dimensions was weighed no more than 6kgs in Economy, it didn't really matter.
There was definitely a restriction on wheeled bags which Virgin, British Midland and British Airways enforced rigidly after the British Midland crash at Kegworth where the CAA recommended a maximum 5 kg limit as cabin baggage was responsible for most of the fatalities in the passenger cabin.
Certainly, in 1997 Virgin was still rejecting wheelie bags, but BA started to relax and so did BMI. The legal culpability for passenger injury from insecure overhead items has also legally shifted from the airline to the bag's owner!
Commercial pressure by EasyJet removing any weight restriction has led to the free for all we are seeing now!