Debit order amendment

I've had a life cover policy with OM for the last +-6yrs with my d/o going off on the 20th of each month with no problems. In early Sept this year I contacted them to amend by d/o date to the 25th because of change of employment and therefore salary date. I was told it was successfully changed up until the 20th Sept when I see an OM debit order going off on my account for a different amount than my premium. I contacted them and was informed that the person who assisted me only changed a certain portion/benefit of my premium and it would be sorted out for the next month's premium. I understood and believed it was human error and it was fixed up until it happened again in Oct and in Nov and each time I call I'm informed it is now fixed for the following month. Through my broker I received a reference number for my query *** on the 27th Nov and I would be notified once it was resolved and I heard nothing since I then queried this myself on the 22nd December because it was still not resolved,I received an automatic response with the promise that someone would call me within 48hrs and ***. No one called. I'm very disappointed in OM and want to cancel my policy.