First of all check your meter and work out how much you are actually owe them,(all rates can be found on each company's website) Then ask for time to pay if it is correct. Ascertain when your contract with them ends. There is a standing charge of a few pence per day, and a rate per unit on top of that.
Once you've paid it all off, change suppliers
I have been with Octopus for about 3 years. Every so often, and at least once a year, I check the rates offered by each company and have never found any better than this. They just ask you to log in and enter your readings each month.
Octopus don't have any exit fees, which I think tells a tale, and says it all
and if you use a link from a current customer you share £100 between you - credited £50 against each account.
They are a WHICH recommended company too
Here is mine if anyone fancies changing to Octopus
https://share.octopus.energy/linen-ant-47But basically the advice is to calculate exactly what you are owe them and sort it out.