That's why you don't do a direct debt that way, as they are guessing what they expect you to use which is usually no where close half the time.
You do what's called a
Direct Debit Whole Amount Monthly (you have to call them to set it up as its an option they don't tell people about), it gives you the direct debit discount (roughly 8% cheaper compared to paying bills as they come in)
The way the whole amount one works is the same way you pay your bills now (as you get them), you get a bill every month for what you have used based on your readings/SM readings and the amount is auto deducted from your bank as a direct debt 10 days after the bill has been issued. Or you can pay the bill straight away when you get it manually, as long as the account balance is £0, a DD will not be taken when it gets to the 10th day. This still gets you the DD discount
Works much better as you cant be charged too little or too much, you pay the exact bill and still get the benefits of the discounts. Make sure to compare your
UNIT RATES however as that is where the discounts (if any) are. Keep in mind, there have been price increases recently and one coming in September.