If you were billed by Telkom for ADSL and by Afrihost for ADSL for the same time period, then send a copy of your Telkom statement to Afrihost and ask them to calculate the correct adjustment. You'll have to make circles and things around the relevant bits, because Telkom bills are inspired by SARS for indecipherability.
It is odd that you are R1000 in the red - the most expensive ADSL line Telkom sells is R599 per month for a 40Mbps line. If you did happen to order the installation of a new line, that might explain how they managed to get to R1000. Afrihost could set the speed of my line without moving things back to Telkom - I don't know why it's different for you.