Well as part of my preparations for moving house I’ve checked out how to move my broadband. Answer: I can’t I must cancel my existing service and take out a new order for my new property. Complete with new connection charge of £69.
So I will be leaving Metronet, it was a good service, but not as cheap as they make out.
Edit 11/12: Actually perhaps I was unfair, Metronet seem pretty open and honest about how their service works and how it is affected by BT’s broadband licensing and their (lack of) service. A quick call to BT confirmed it was BT who were the problem over the transfer.