Then you should give serious consideration to cancelling any bank arrangements simultaneously.
My recent spat with VirginMedia (elsewhere here) is a case in point. By cancelling the DD I made sure that they could not take money to which I thought they were not entitled. It puts the provider on the back foot, chasing you, rather than you hassling a large, insular organisation for a refund (or chasing your bank under the DD mandate rules).
It's a tactic I have used on many occasions, have yet to regret it. A personal banking service to whom I can send an email requesting the cancellation (or a telephone banking service) aids a speedy execution.
One word of warning though. Some companies (energy providers) have a clause to cancel discount arrangements if you do this. As ever, read the terms and conditions, and make sure.
Take control, and use the weapons at your disposal with big business:
* your custom
* your money
* your use of social networks to publicise your complaints.
Oh, and if they get it right, publicise that too!!