Bad service

I have been paying for an extended Toyota service plan with them. On month I changed jobs so I had to change all of my debit orders to the last day of the month as apposed to the 27th of each month. That was done swiftly and with no hassle. This year I changed bank branches and called them to tell them the new account and to tell them that I need them to do a new debit for an account that did not go through the in December. They agreed all and well. When they debit they debited me double but I didn't have that much in my account but what was enough for one debit sum. I have sent email after email and they only reply when they feel like it (like after a week of nothing ). I demanded that they pay the arears amount that the bank owed me for their errors, now they fill me that the day I had changed my debit date they hadn't debited for the month I changed it on. All good and well but why would they not tell me my account is in arears so I can make payment arrangement for both debit orders. I'm due for a service next month and I feel like they are in cohorts with Toyota to make me pay for the service whilst I been paying a monthly fee. Now my problem is an accounting department surely cannot be making these mistakes we take these debit order options because we can't pay a huge amount on a given time and this makes me upset because this is another case of the rich taking from the poor. After all of this I'm inclined to just stop the debit orders and save for a service if this is what they do to people.