Hi Julian,
Your question isn't a simple one to answer without a lot more infoamtion I'm afriad but I cna say that the chances are likely that there are things that can be done regarding the balance.
The first thing we need to do is get a handle on the specifics of the situation. I would ask for copies of all statements from both British Gas and Powergen/E.on from that start to the end of the accounts. Once we have these look to see excactly what and how you are billed. Things to look for intially from the Powergen/E.on are any references to final bills, do they cover both gas and electrcity and what dates. From the british Gas bills we'll be looking for what fuel/s they are actually billing you for.
It's possible that while you requested British Gas take over both fuels they simply didn't for one reason or another.
In the meantime I would also ask Powergen to place a hold on the account while you look into the claims. If all goes to plan the DCA should stop contacting you (may take up to 14 days form your contact with them). But it has been known for contact to continue, they'll claim they're acting on the best interest of their clinet but it ultimately comes down to them wanting their commision for getting you to pay the bill.
Once we're able to see what and who are billing for what we can then consider what's best to do moving forward. At this stage there's just not enough infomation.
A further thought, to speed thigns up you may be able to register for internet access to both these accounts, and view all previous bills sent to you. The accounts still exsist even if there's no on-going supply and this might make retrieving the infomation you need quicker.
Post back when you have the bills or access tothem and we'll go from there.