Past due credit ,right or wrong?

well basically i owed a gas/electric company some money on my bill i could not afford to pay the bill so thye installed a meter so i could pay the debt whilst using my electric ,now i changed companys but was still paying the debt on my meter anywasy i have paid all except 39.00 but i have my old electric company who i owed the money too sending me letters to say i still owe naerlly most of the debt i do not , they said because i changed company my new supplier would have the money infact it was not going to them ,my new supplier say im not in credit with them now i have a debt company called past due credit hounding me for the money i have already paid ,i do not and will not pay twice, i reluctantly offerd to pay in small instalments as i am a single mum to 3 small kids on benefits now that was ok but because i didnt have a lump sum to put down today to activate the instalment account they told me thye was taking me to court?? now i do know if i am offering to pay something even though i should not have to that thye cannot take me to court ,it is not my fault that i didnt have any money today to give them infact they were very rude, i was told they cannot take me to court for such a small amount and if im offering instalments??? thanks in advance