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.