Thanks everyone for the replies.
Yes the line is still active. This is what I don't understand, the number was originally a BT number, the line is still active, I thought that was all that's needed to be able to migrate your number?
Keeping your number in 2020 shouldn't be this hard. I wouldn't have moved if I knew this was going to be the case. So am I locked to Virgin if I want to keep the number I've had for forty odd years? Seems absurd.