Please please can someone advise me regarding an outstanding debt I have with EDF Electricity.

Back in November I agreed to go on a pre-payment meter as I had run up a huge debt with them for £1400+. It was agreed over the phone that the pay off charge to be added to the meter would be set to £7.50 per week, as that was all I can afford.
Obviously since the meter was installed this has (or so I thought) been getting paid along with the electric I've been usuing since, it's been working out to about £17 a week which I can just about afford.

Anyway to my horror a short while ago I checked how much credit I had left as I earlier put a tenner on it which had to pay back some emergency credit, to see that it had almost al been used!!
Decided to open the letter I got from them today, to see that as the debt is still outstanding to the same amount as back in November, they are going to be putting the pay off charge up to £28 a week, which must have been triggered when I put the credit on earlier today!!!!!

I really cannot afford this, outgoings way surpass income at the moment as it is, yet these fu**ers know they have me over a barrel as I have no other choice to pay it!
Now surely if the meter wasn't set correctly when it was installed that isn't my fault and is there anyway I can protest about this and get it altered?

EDF aren't oopen again until after the weekend and to say I'm freaking out is an understatement, I'm going into hospital on Monday to have my 3rd child by C-section and really could do without this stress!

Lisa