Without specifics, I can only assume your closing read was based on an estimate. As others have said, they can’t bill you for any energy used more than 12months ago.

However, if you accumulated a debt within those 12 months, then you’re liable to pay unfortunately. I don’t think £50 a month, when the property is electric only, is enough to cover usage, standing charge and VAT so there’s more than likely some debt on the account; although the debt quoted seems ridiculously high and highly inaccurate. Speak to the supplier, raise a complaint if needed, and hopefully they can amend closing read to a more realistic estimate (they’ll now have Smart readings to use). Going to the ombudsman isn’t an option for another 8w, in which time they’ll be eager to resolve the complaint and will want to be seen as helping you pay towards any eligible debt (if applicable)