Just called EDF about this – they say it's no problem to change back to economy 9, but the hours they quoted me as off peak were
3 – 5
6.30 – 7.30
14.30 – 15.30
22.00 – 23.00
That's only 5 hours! I'd be better off with economy 7, even with the horrible times!
Charges were a fixed daily charge of 29p, then 12.31p for the first 1500 units per quarter, 12.something p after that, and 5.20p for off peak. Don't know if this is the same for other people.