NPower - 2 years of complaining and still no bill

Hi all, I'm a newbie here, and would be very grateful for any wisdom you can impart.

My family and I are in the process of paying ourselves out of debt, and everything is carefully budgeted in our household. But we are having a major issue with NPower.

Back in 2015, we were accepted for a GreenDeal plan to have a new boiler installed. Not the one for low incomes, but we got the boiler, and we signed up to pay for it monthly, using the reductions we should expect to see in our heating bill. Our NPower account was amended to reflect this, and we went forward with a new bill, showing Gas, Electric and GreenDeal on it. They told me that the new monthly payment for us would be £76, and this was all sorted.

Until November 2015, when I didn't receive a bill. They were taking their £76 each month, so we had a credit balance showing, but they didn't actually take any money out of our account. So January 2016, I rang them asking for a bill, which I was assured I would have within 28 days. Only a month later, I didn't have one. I chased this up monthly for 12 months.

January 2017, I rang Citizens Advice, who put us in touch with NPower's "Executive Complaints" team, where I was assigned a caseworker, who was lovely, and advised she'd have this sorted for me very soon, and advised that it was the GreenDeal that was causing this issue. Then finally, in March 2017 I received a bill, but it was only upto November 2015. So I chased the caseworker, who said she would issue a manual bill. And she did, but she said it didn't reflect my actual account, which would likely be impacted by the "back billing" code of conduct and various other things (so it wasn't very helpful). She said she would continue working on it.

Then October comes around. By now, my account is £2900 in credit, and I receive another manual statement, showing a balance of £900. So I rang up, and was told that this would be on top of the £2900 in my account, but again, it wasn't accurate and unlikely to be anything like the figure that would appear on my actual bill.

At this point, I started to panic. There is no way I can find £900 on top of all the monthly expenditure, so I asked her what to do. She basically told me to start saving up for when the bill comes in (which she expected to be in December).

So, now we are in January, and still no bill. Rang Citizen's Advice, who told me that there's nothing they can do except refer me back to NPower.

I've just been in touch with the Energy Ombudsman, who have opened a case, and told me that I need to collate my evidence for them. Other than the screenshots showing that there is no recent bill, and the regular meter readings I've been sent, I don't have anything to send. I thought this would be sorted, and trashed the manual statements when she told me that they weren't right.

Does anyone have any experience of how this Ombudsman service works? And of anything like this happening before?

Sorry, I just noticed how long this is - I've been working on this complaint for 2 years. Sorry.