I changed energy suppliers earlier in the year (EDF) and pay by monthly
direct debit to get the cheapest tarrif. My gas payment has been 52
GBP, and my account is currently 22 GBP in debt.

Today I received my latest bill, informing me that they have reviewed my
monthly payments and based on forecast usage (ie over winter) they are
going to increase my payments to - get this - 120 GBP.

This strikes me as utterly ridiculous and I fully intend to whinge, but
before I start raking out old bills and trying to tease out me winter vs
summer gas consumption, does anyone have a rule of thumb as to how these
payments are or should be calculated? Maybe EDF are actually on target,
though seems unlikely to me?

(I wasn't with EDF last winter so they have no clue as to what my winter
usage will be. We use gas for HW, CH, and a hob).

Thanks
David