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Thanks all! The previous tennant is a acquaintance of mine, and he has been using a prepay meter the whole time, so can't have got bills...He also said that you can't be in debit with this machine. He said it sounds extremely high, as he was normally paying around 25 per month or less in winter!
I have now put another �10 in, and the machine said "18 KW"...
I've no idea what that means, but if it runs out quickly again, I'll be phoning Phoenix. I hate being ripped off! lol
Oh, and there is no such menu on the screen, Bravejordy, nothing happens when you press red, and black is only for updating the total when you put money in....