NPower - Why, Thou May Have a Solution!

As an update to this little saga, NPower have called to tell us that they were in the wrong to sell us the product in the first place and that our meter isn't supported by them. There are two possible resolutions - either to go back to Eon with a cheque from NPower for the amount we've paid them to date or have our meter changed over to one they can support (which I don't think they want to do as this will cost them money in contractor charges etc.).

I have just had a conversation with the Complaints Department at NPower who's remedy for our problem (and complaint we have lodged with them) is to pass us back to our previous supplier Eon, who incidentally, were just as incompetent hence our reasons for selecting NPower in the first place, if that all makes sense.
Our basic argument is that NPower sold us an electricity supply contract last year promising us a monthly bill of £126. They told us this would not change and we would be consulted when this had to be reviewed. This was more than £50 less than we were paying Eon previously. We were reeled in.
Despite paying this, we have also incurred a debt of more than £800 in usage charges. We were informed of this at Christmas and on January 1st, NPower emptied our bank account to pay off some of this debt. This left us in arrears with our mortgage provider and our council.
Having spoken to them this afternoon, they now do not know when we joined them and have obviously been unable to retrieve the recording of the telephone call we originally had with their sales department. Their suggestion for resolving the problem is to pass us back to our previous supplier Eon. If this includes writing off the £800 in spurious usage charges, I'll be more than happy to do so, but I doubt it very much.
Apparently, according to Eon, we also have an Economy 10 meter. I had no idea there was 'Economy 10'. The only conversations we have had with Eon have been about Economy 7. Because of our 'odd' meter, we do not fit into any of their tariffs so we appear to be paying more than we need to. Why weren't we told about the meter right at the start?
I'm already preparing my letter for the Financial Ombudsman.