Phone charges continuing on a phone transferred on 12 December.

I was notified by my new home phone provider that my number was transferred from Virgin on 12 December. I have been billed by £19.00 per month by Virgin for January and February because Virgin say the number has not been transferred. When I phoned Virgin to complain about this billing I was asked to get in touch with my new supplier regarding this, as Virgin say the number was never transferred. My new supplier tells me the line is fully operative and has been transferred successfully on 12th December, and there is nothing they can do to sort this out. Virgin say it is down to my new supplier to sort it out as they are adamant they still have the number. I am stuck in the middle paying both companies for the phone. How can I resolve this when each provider says it is down to the other to sort it out!