EDF are absolute muppets, I have had bad dealings with them in the past and they are clueless. As ConmanKiller says, stand by your rights, and speak to the CAB and EnergyWatch to get a case started on this. Dont accept anything in your name, challenge everything, refuse to do anything unless you get it in writing first, and generally make it as difficult as possible for them. Also you might want to outline your case in a letter sent recorded delivery to EDF energy, explain the Energywatch as above, and suggest that you're happy to pay your last year's bill (suggest a figure to them and offer to pay it there and then) include a record of all contact you've had with them, and suggest that any further demands for the full amount will be construed as harrassement and you will seek further action in the courts to resolve the matter.

Good luck and please keep us posted on events as they unfold.