Hi laurenelisabeth
Firstly I do really sympathise with you here but not sure if what i write will be of some help.....
Clearly you both lived in the flat and were responsible for joint energy usage and more importantly payment of the fuel you had both been using. Especially with the birth of a new child especially gas usage increases due to ensuring the property is kept warm for the newborn.
EDF may seem harse in their actions however they are simply ensuring payment of the fuel they have provided, as you mentioned, your partner was abusing their trust by providing false meter readings for both accounts, but one of these accounts were in joint names, making you both responsible.
All that said, however, I would contact EDF and ask for their complaints proceedure and honestly I wouldnt get irate or angry at the first person you speak too as rest assured they place detailed notes on your accounts and really wont want to help any person like that, I would simply ask to speak to a manager and ask if there was any way possible the original accounts could be both be placed in joint names and billed and closed up to the date your partner moved out (October 2007 ?). This would then half the debt you are faced with as I think legally if joint names were on one bill then its fair right that it should have been on both bills. I would then ask them to only charge you for your usage in your own name from October 2007. I would ask them to move over the half the electricity & gas debt which leaves the other half to be footed by your ex partner, offer to give them a forwarding address to help with debt recovery, and realistically give it to them, either his real address if known, or last known address or even his parents, as no company would possibly agree to this if they havent got a secure address they can at least begin trace follow up on.
When your balances are transferred ask them to do an assessment on your energy usage, both gas and electricity from October to current date. This gives you an idea of your true energy usage and what you should be paying for your ongoing usage, then ask them for payment arrangement on the debt bfwd, see if you can get this over 2 or 3 years or possibly ask them to install a PrePayment meter to help you budget your usage thus ensuring you dont accrue more debt because you cant meet your ongoing usage aswell as your debt.
Hope this helps and let us know how you get on