I just came across this thread. I'm pretty sure the communication VM sent me last year said that unused credit would not be returnable. Now I am reading that we could have reclaimed it after all - but it is now too late. Our VM SIMs went in the bin when we purchased new SIMs from Tesco Mobile and the VM ones no longer work anyway. The day after the cut-off, my wife (who VM didn't actually inform at all) went to make a phone call and got, "SIM registration failed". That was the first she knew about it.
We are both very happy with Tesco Mobile, but it is extremely sharp practice of VM (not to put too fine a point on it) to have stolen customers' money like that. Why not refund it automatically? I lost £10 and my wife lost £53.
Are you sure it's too late? We were told by two Virgin people that it was impossible before somebody on here helped. You're right that a communication from Virgin said that the credit had to be used up, but at the same time the website was saying that you could reclaim it. When the story of Virgin stopping PAYG broke, it said in the press that you had up to a year to reclaim the credit. I assume they got this info from Virgin.
I already lodged a formal complaint with Virgin before we got our money back. It might be worth considering that. If necessary it can be escalated all the way to ADR. I'm no lawyer but it seems very dubious practice to me.
Hope it works out. Note that we managed to get a refund even though we'd already ported the number to a new provider, so I'm not sure why you shouldn't get yours back too. For me it was a matter of principle as much as anything.