Had the same thing when I first moved into my flat — got a bill from British Gas for £800 (?!?!?!) for the month we had been covered by them before we moved to another company. Apparently the previous tenants provided ridiculously low readings so they got a lower bill and we were stuck with the excess.

We fought it with the ombudsman and won. Keep fighting it with the council and take it higher if there's an avenue to!