Hi Andyddg
I was also with Iresa, and am in the same position as yourself, when I received my final bill, my gas consumption for July was 1400 cubic meters, I don't us that in a year!.
I clicked on the disagree button and supplied them with my meter readings for Gas & Elec, and also queried the electric readings as I have had a smart meter fitted and have solar panels, yet according to the meter I have used 1200KWH in 40 days, something not right there, and have been informed that the meter may not be set correctly and is charging me for power generated as well.
In response to the bill they have sent me an offer of £97 ish, but haven't attached any bill or breakdown of how this was arrived at, so I have requested it and also asked where my payment taken in August has gone?
My Iresa account is still active, and I have downloaded the last 12 months of bills, if you can do that it will give you an indication of your consumption and allow you to compare what they are saying you have used, and also give you the ammo required to challenge them.
As for the DCA threat, I think it would be a foolish move on their part whilst the final bill is in dispute.
Hope that will help you in some way.
Paul9