I too have had an absolute nightmare with them and am sick of them.  I have been paying regularly every month for the past couple of years with them, I wanted to leave because I always received incorrect letters, then suddenly today received a letter saying we were overdue £3,500 and need to make a payment urgently.  I was on hold for over 20 mins before being transferred to customer services, who I couldn't understand and the line was poor.  The only thing I could make out was he said my bill was estimated and that he would send out a statement.  Now there is no way on earth we have used £3,500 in gas and electricity, we have a standard sized house and work all day and are out most weekends and have been paying every month.   I have asked to move suppliers but they say that I can't until the arrears are cleared and these are rocketing each time I speak to them.  I moved to them to get cheaper gas and electricity but it appears their rates don't exist now so I'm just going to be getting myself more and more into debt its really stressing me out and don't know who to send this to next??