Changed d/o dates but still debited old dates.

I changed my debit order date to the 15th because I changed jobs and received sms with a reference number to say it was done. <br> <br> My debit order went of f on the old date (1st) regardless. When querying it they refuse to reverse the transaction black mailing me that my credit profile would be affected if I reverse the transaction and I just have to accept the way it is. I don't understand how they can get away with this I have the reference numbers to prove they said that it was done and now my cash flow is messed up until I get paid the 15th. Who can I contact to put a complaint in against the bank?<br> <br> They changed one vehicle's date only and I requested both so they clearly made a mistake and admit to it, but just refuse blatantly to reverse the transaction and take the money on the arrange d/o date.