vodacom obviously losing the plot

I received my bill payable at the end of May yesterday. It indicates that I didn't pay my bill at the end of April. However, I have proof of payment that the debit order for the end of April indeed went off. Considering the terrible service I have received from Vodacom over the last couple of months, my patience is wearing thin. I suggest that this issue be resolved and I am sent a revised bill for the end of May, since I willl not hesitate getting legal advice if this billing error impacts on my credit rating or blacklists me.