Incorrectly billed, freezing my bank account

I changed my billing details with Discovery on the 6th of April and arranged for the first debit order to go off on the 10th of May as I only get paid in the 1st. This meant that the first instalment would have to be a double instalment. <br> <br> On the 20th of April the debit order incorrectly went off, taking the last of the money in my account and sending my account into overdraft. Now all my bank accounts are frozen, financially I'm dead in the water. I found this out at spar's checkout when I was trying to pay for groceries, embarrassing! I had to walk away empty handed. <br> <br> Discovery said they will only be able to refund me in 7 days. For such a colossal ***** up I'd expect them to rectify the matter immediately! I'm now liable for extra bank charges for going into overdraft. This will undoubtedly also have a negative impact on my credit worthiness.<br> <br> I would recommend prospective clients to think twice about signing up with them, I'm considering cancelling my contract with them. For a company that should be looking out for my best interests, they have dealt me a very low blow.