I've just managed to get through to a really good chap on the phone and for anyone affected by this it appears to be good news.
VM are only recently aware of the issue, but they're looking into identifying the customers affected and fixing it. It looks to be possibly related to the serial number of old boxes (I know I've seen a few posts of people saying they'd returned boxes which is what I'd done).
Unfortunately the fix is to wait for them to fix it and then they'll automatically refund everything owed. In the meantime we have to keep paying for it unfortunately. Still good news really.