Do you mean your DD is equal to your quarterly bill or equal to 1/3 of your quarterly bill?My DD goes out monthly but my bill is quarterly so whilst I have ~£500 credit now I know that is going to be wiped out next time they bill me. To be fair to them my monthly DD that has risen quite substantially is pretty close to what I pay quarterly.
I presume the former in which case why don't you reduce it based on your proven consumption?