Chased for a debt I have already paid

[flaybourne]

I have recently been chased for a debt I agreed a partially settlement on over 2 years ago. This shows on my experian report has satisfied and Experian advised me to send a copy of my report to the debt company. I then received another letter asking me to contact them urgently. I rang the company to be dealt with by a very rude woman stating that the experian report means nothing to them and I have to show a receipt for the payment or pay it again. Im worried that I cant remember how I have paid this and think it may have been cash payment and I have no receipt but I definately know it has been settled. I really dont know what to do now and the company they bought the debt from dont seem to want to help without me giving them a reference number. What can I do???