I had the same experience after noticing the IRESA tariff, and wondering why they just couldn’t provide a peak and off peak tariff and use my existing Economy 10 meter. After a long time on hold (c.30mins) (I assume this is their lean business model in order to offer lower rates) I was told they wouldn’t offer it for Economy 10 but I could have the E7 tariff and they would arrange for the installation of a new E7 meter. E10 times suit me better so I did not pursue it any further.
I wouldn’t surprise me if they did allow E10 meters though, I got the impression the Call Centre didn’t understand the issue and just wanted to move on to other things.
Good luck. I am still trying to find a tariff to rival IRESA which I can actually access!