Do you have any evidence that they were more expensive? Certainly since I started working in the energy industry those with PPM have been charged the same as those paying quarterly on receipt of bill.
It seems more expensive to some as during winter you really do have to feed quite a lot of money into these things. It's always been my belief that if the whole of the UK were forced to use PPM's you would see consumption plummet overnight - a lot of people with credit meters are incredibly profligate!
Those with PPM's - edf and British Gas will change them free of charge to a credit meter, where you can take advantage of the 6% DD discount (subject to passing a credit check).
We will have to disagree but first hand experience dictates otherwise.
It is some years since i had prepay meter (5+ & i agree things may have changed) but my bills at the time suggested the tariff was higher than having a "normal" meter. I certainly paid more over a measured period (one standard billing quarter) than i did on a "normal" billing system - AND this was even after moving into a smaller house.
Once you have them it is incredibly difficult to change to a standard meter - despite what is suggested - and it is very difficult to persuade them to change your tariff. I will agree that if you have a good record of payment and you move to a house with a key meter they will change it. If you dont they wont. As for a credit check, most people on key meters wont pass one!
If you get into debt they take the money at source meaning you have to put extra on your key to make the lights work. No arranging a sensible payment plan like you can on your standard bill!
Finally you have access only to the open tariff, preventing you from accessing any deals in the market, you have no option to fix or change easily like you do on a standard bill and you often feel like a prisoner!
AND it always runs out at 3am, when no shops are open!
(the gas one was worse than the electric btw!)