For the benefit of anybody who is unsure whether their build has been finalised and transferred onto normal domestic electricity rate you can check in the top left hand corner of the bill below the ΔΕΗ logo. The first line will say either ΕΝΑΝΤΙ or ΕΚΚΑΘΑΡΙΣΤΙΚΟΣ depending whether it is an intermediate bill or a final one. Below that if you are on domestic electricity it will say Γ1, or Γ1Ν if you have a night meter fitted. If you are still on builder's electricity, i.e. business rate, it will say Γ21.
I checked back in my records and I received two bills on Γ21 after I moved into the property. Both had the ERT charge on them. Neither had the council tax charge on them and I was NOT charged any back council tax. It simply started at the normal rate on the first Γ1 bill. In my contract with the developer it specified that a night time meter would be fitted. It took quite a long time to get ΔΕΗ to do this after I moved in and also to transfer the meter from its temporary position on the house wall to a box on the outside of the garden wall. I guess it is possible that my property was classed as not complete, even though I was living in it, because ΔΕΗ had not fulfilled their obligations. It is also possible that back council tax was not charged for that reason. That is why, in my earlier post, I said that you "might" be charged for back council tax.