Interest Charged due to Bank's Falut in not executing debit order!

I noticed a charge on my account for interest over the past two months. Mystified, I called the bank to find out why. Turns out they are charging me interest for a missed payment that was due to THEM NOT EXECUTING A DEBIT ORDER! Tell me how that makes sense? Through no fault of my own I 'missed' a payment? And I must now spend 3 or 4 months paying interest when I did absolutely nothing wrong??? I settled the account the day after I realised the debit order didn't go off which quite honestly is the best I could have done. Spent over an hour on the phone speaking to various of their so-called 'bankers', in the end the best they could do was tell me they would submit the query to their complaints department. I'm very disappointed. Will be taking this further.