Hi Spellgirl,
I work for E.ON so may be able to give you some advice.
£45 per month sounds low for a duel fuel account, you have probably been underpaying for quite a period of time, especially if you have a debt on the account.
A Direct Debit should take your projected annual usage and divide it over 12 monthly payments. You would see a credit build up over the summer to cover high winter bills.
As you have a debit balance it sounds like the Direct Debit has not been high enough. This means that the new suggested figure will be made up of your correct monthly usage plus a figure to repay the earlier underpayment.
I'd ask E.ON how the direct debit has been calculated, and how much is the repayent of the balance each month.
Firstly check the bill is based on accurate meter readings and is correct. If the usage is correct the Direct Debit will have to remain higher as a reduction would mean you continue to underpay.
If changing supplier is your intention generally the balance will need paying first to ensure the transfer is not objected to.
Brian