I am having a similar problem with Scottish power, we have only been with them since october after being with Npower, and relatively a happy customer.
We were quoted a saving based upon a realistic consumption and annual spend of £17 per month saving against npower.
Everything was great for the first 3 months at £63 per month, then they announced that the direct debit was almost doubling to £110 per month. We then phoned them to let them know we werent willing to accept this, whilst on the phone, they announced that the figure was actually closer to £330 per month for 3 months when our debt was only £400 for a quarter !!
I asked them "why are you wanting £990 over 3 months when we only owe you £400?"
She couldnt answer the question, and just said this is how we do it, I went ballistic, and told them a few choice words, and since then, things have got really ugly. debt collectors etc, I am now paying £100 per month.
Since all this has been going on, I have been monitoring my gas consumption very closely, I have noticed that my gas usage is so minimal that i thought i should ring scottish power, they then said "well yes it is lower, so i will reduce it to £180 per month.
the company is a joke