sweetpeach - the point is that even if you over-pay a little bit on the monthly DD amount, it all evens out eventually. So let's say you set your amount for the UK average and then make all these improvements, causing you to use less fuel. So when the next bill after the improvements comes, you've probably overpaid somewhat, but all this does is put your account in credit and likely trigger a small, automatic reduction in your monthly DD amount. Or if you want to keep your acct in credit for periods of increased usage (let's say winter is coming up), you can leave the DD amount as-is and then you've got a cushion for when cold weather comes.
Does that make more sense? I think energy companies differ slightly - if we underpay, all Scottish Power do is raise your monthly DD amount to cover the amount of "debt" on your acct. I think other companies may actually chase for a one-off top-up payment instead. For instance, our DD was obviously slightly too low to cover the really cold winter we just had, so our monthly bill was raised £13/month to cover the deficit our acct is sitting at. But after next quarter, this'll prolly be recovered and our energy use will decrease with the coming spring/summer, so the DD amt will fall slightly. It's all automated triggers with ScottishPower, but other companies may not change your DD so often - ours is adjusted ever quarter, but not sure if htat's normal or not.