The bill is over a year old and, as far as I can tell, they will not be able to claim from 2004 but rather April 2007 as from July 2007 members of the site (link below) agreed not to pursue bills more than a year old if the company was at fault for not supplying the bill/ getting the correct readings etc - failing the code of practice in other words.
Guidelines and info are on the Energy Retail site which lists terms and conditions of its members and E-On is listed as a member:
The Energy Retail Association
The company has this code and practice section:
Key Facts
In 2006 the ERA introduced a new Code of Practice to help customers by clearly setting out what they can expect from their energy supplier, including:
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Clear, accurate, informative and timely bills and statements
Support and advice on monitoring energy consumption
Support and advice for those having difficulty paying their bills
Contact details for raising questions and issues with suppliers
Therefore I suggest you take a look the site and follow it with a very strong letter of complaint stating they are breaking their code of practice.
I think however you may have to pay for the bill from April 2007 and you can ask for a year payment plan to pay this in.
If anyone else know any legal issues whcih can help clarify this, as I do not have any knowledge of legalities.
Hope this helps and good luck.