@Cmtc21 Hi, that is a really funny computer glitch. It means that if you request your refund there is no high enough monthly payment to get you back on track for the winter. Of course that is silly, if you paid say £500 a month that would easily cover you, but I guess it means in practice that you cannot have all your credit back and still stay on a DD system.
You could ask to go on quarterly cash billing, and save the monthly amounts in your own bank account, in that way you could have all your credit back. If you stay on DD most people have a high balance going into the winter, that is the way the smoothing works, you pre pay in the summer for winter usage.