barmycharlie:
Well its already after 8am so maybe you already have your answer but in that case I'll answer for the benefit of others.
There is lots you can do to ensure accurate payments.
(1) Whenever you get your quarterly statement always replace any estimated readings with an accurate customer reading.
(2) Ask your supplier what your annual consumption has been.
(3) Go to a price comparison website and plug in the annual consumption data for the tariff you are on to see what the actual annual cost is.
(4) Divide by 12 to see what is an accurate direct debit payment.
(5) Armed with this information tell your supplier what you expect the payment to be and ask for any surplus to be refunded. The amount of the refund will be subject to some negotiation because its reasonable for a surplus to maintained in summer to meet the heavier winter consumption.
(6) Look at the results at (3) and decide if you would get a better deal by switching to a different supplier (assuming you are not in a fixed term)
Switching (using annual consumption) is a guaranteed way of getting all your surplus back if you are not satisfied with the amount offerred. Also if you switch using your annual consumption, accurate direct debit payments should be set.
Regarding changing from fixed monthly direct debit to quarterly billing it may or may not be possible on your existing tariff. You possibly receive a discount of up to 6% for fixed monthly direct debit. Only you can decide but you can check the extra cost of quarterly billing over fixed monthly direct debit using a price comparison website.
Later edit:
Here is the BG help page on too high direct debit payments. Note the aim in bold.
https://www.britishgas.co.uk/HelpAndAdvice/TypeID/?Id=754&Sec=R