Guessing with the lack of info you may have been on estimated readings for a while and then an actual reading used on change over much higher than estimates, however this would lead to bill with old supplier not the new one. so still doesnt make sense.
What I will say is they are supposed to give you a reasonable amount of time to pay off a large balance, as I understand it 12-15 months without income check, if you want longer you may need to go through finances with them.
With such a large bill it might be worth asking for meter to be checked, will be threatened with a circa £80 fee, but personally I dont believe the story that the vast majority of meters out there which many will be decades old never checked are not faulty.
For reference when I moved in I had really high gas readings (and bills), I got it checked, there was no admittance, but basically bill was written off, fee waived and new meter fitted which gave much lower readings, make of that what you will.
I had a smart meter fitted last week and so far average readings are close to 40% lower vs my old meter which is insane, in this era thats a LOT of money.