Debit Order Date Problem

Vitality is extremely inconsistent when it comes to their monthly debit order. I have been struggling for months now to get them to debit my account on the 25th or the 1st but every month they are still debiting on the 9th or 10th. In November they didn't do their deduction and with no communication just took double the next month. I am really tired of phoning you people every month only to be told on the phone that you have changed my date but come end of the month nothing has changed. I DO NOT WANT TO HAVE THIS QUERY AGAIN. YOU NEED TO SORT IT OUT IMMEDIATELY!!