Who do i complain to?

yesterday i recieved a letter from moorcroft debt collectors saying they had been instructed by severn trent to recover the �160 i owed them. i was completely unaware i owed any money as i had not recieved any letters off them saying my bill was overdue, i rang severn trent and they said they had sent 'several' letters, non of which i recieved. do i complain to royal mail for me not recieving the letters? do i complain to severn trent for not phoning me to make me aware i owed money? im really hacked off about it especially as moorcroft are being really nasty to me like i just dont want to pay the bill, im prepared to pay if i owe it but they are being a*se's and want payment in 24hrs which we dont have as we dont get paid till the end of the month!