Scottish Power prepayment 'debt'

0161a

I had duel fuel from Scottish Power, the gas meter was a credit meter and the electric was pre payment.
I have a long running history of difficulties and complaints with them. I finally came to an agreement that I could change providers.

The changeover went ahead on Tuesday. Prior to this, I had rang Scottish Power re the money owing on the accounts. The electric debt was only £57 a month or so ago going down and it was agreed that I would clear the debt on changeover, it would be less than £50.

The gas had arrears on it and would take £24 months to clear. ( I had missed payments during a difficult time doing agency work and income was up and down)
I had been checking my online account daily and on the 1st November , overnight, my £57 debt for electricity went up to £240.
The person in the call centre had no idea how and told me someone would ring me.
I got a call to say that apparently the debt goes back to March 2013 when they changed the meter. My online account goes back to 2017 and doesn't show this 'debt.

It gets worse-I opened my messages in the online account and there is an electricity bill for £1600! and a gas bill for the same.

can I challenge this ? is it statute barred or do I approach this problem by complaining about back billing?

Thanks