@chrisScan Hi, you will get estimated bills if you do not send in a reading. Your meters have a keypad on them, press 9 and the meter reading will come on the screen. Make a note of it and put it in online in your SP account.
@Ronco I've been suggesting this to people as an option for a while but I don't know if it would work. It's free to do though, so may be worth a try.
Under GDPR2018 you have the right to submit a Subject Access Request to any company that holds data on you, asking them about the data they hold. They are legally obliged to provide you with that information (with a few provisos), and MUST respond to your request within a month by law. PwC are the ExEn administrators, who have had such a hard time of getting your information out of the ExEn accounts to begin the repayment process, so I would imagine THEY are the people who now hold your ExEn data, as ExEn is now defunct. So I would think they would be the people to ask about what your historic usage and payments were. If not them, then maybe Scottish Power has it.
If ExEn weren't in too bad a shape at record-keeping, your data on usage and payment history SHOULD be there to see and once you've got it (if they have it to give you) you can marry it up against what SP say and see if you agree or disagree with your final bill.
If you google 'the ICO', they have a website with guides for submitting a SAR. I say again - I don't know if this would get you the data you want - but it might, and it's free to try, so long as you ask the right company and ask for the right data.
@Ronco By way of explanation @Jonnel is talking about the data protection Act which gives you access to personal data held by others, it is possibly not relevant to finding old invoices, but as he says it may help.
ICO Is the Information Commissioners Office
GDPR Is the General Data Protection Regulation
SAR is a Subject Access Request, which individuals can make to get a list of all data held concerning them
PwC Is Pricewaterhouse Coopers, one of the 'Big 4' UK auditors who are now in charge of the Extra Energy accounts.
It does sound like SP are investigating your case, and I hope you get it resolved with a bill close to what you are used to paying.