My story comes from more than a year ago but still very relevant here it seems!
Upon moving away from them, we tried to pay Scottish Power the remaining balance on our account, about £70, no less than three times. (I must add, we inherited Scottish Power from the previous owners of the house, it was not by choice!) Each time we were told that our payments by card had gone through with no trouble and that our account was up to date.
Lo and behold, a couple of weeks later on EACH occasion we were being sent red letters demanding payment for the amount we had already paid, each letter progressively more forcefully worded than the last.
Naturally I called them up and went mad at them each time (politely, obviously! It's not the callcentre workers' fault) and when we made the payment again, although we were told everything was OK, we found out that our payment never actually went through, despite being told it had.
When the third strongly-worded letter arrived, a threat of a visit from the bailiffs, I had to resort to publicly complaining to them on Twitter before anything 'real' seemed to happen.
To cut a long story short, they responded to my Twitter message within 24 hours and a very polite and apologetic chap called me on my mobile to get it all sorted. It turned out that the problem was entirely at their end and although none of my final payments actually went through, they wrote off the final bill as a gesture of apology.
Bless them. They do try ... If only the whole of the company was as good and professional as their people who deal with customer complaints. They really need to sort things out before sending out threats of bailiffs to people's houses. Some people are a lot more sensitive and more easily scared of threats than I am (having previously having dealt with parking companies before) !