After writing a long email to PlusNet's board of directors and complaining about their lack of competence and support, they phoned me and tried to sort out my problems.
They offered to cancel my cancellation and attempt to work with me and my new ISP to get me transferred, seamlessly, using MAC Keys.
Unfortunately the cancellation had gone in to their provider, Tiscali, ahead of schedule and I had already been disconnected.
So I let Demon know that the phone line should be clear of any ADSL and that they could proceed with their installation. They quickly confirmed that the line was clean and that my installation request had been put through to BT.
Within 2 days I was online.
Demon are prompt, their support is brilliant, the connectivity is fast and extremely reliable. If anyone is using PlusNet and reading this - I urge you to switch ISPs.
PlusNet are truly hopeless.