I have an Avant and Virgin so can answer this one!
Your Avant needs to have the Set Top Box Controller (STB-C) installed AND a software chip that includes 'Pace 4000/4001' in the list. If you don't have either, they can be purchased and retrofitted quite easily yourself (I have a spare set of both).
You'll then need an IR lead coming from the back of the Avant and stuck to the front of the Virgin box. You'll then be able to control the Virgin box via your Beo4 with no problems, and the same will apply in all link rooms too.
The problem I have is trying to control the Virgin V+ box (thats the Hi Def one with hard drive recording capability). With this box, even the latest STB-C software for the Avant will NOT control it properly. You can do all the functions of a normal Virgin box, but cannot access anything to do with your recorded programs etc (i.e the '+' part). The only solution to this is to buy the Beo5 which will control the V+ directly rather than via an STB-C. As mentioned on this thread, this won't help controlling the V+ box in link rooms though.
I guess the only solution for me is to wait until my contract expires and move to Sky!
Hope this helps!!