Council tax benefit overpayment, please help?

I received a letter on Friday dated 10th January stating that my council tax benefit had been reviewed.

It states that the 'old' amount I was receiving was £19.47, however I am now to receive the lesser sum of £14.60 per week.

As a result of the revised decision there has apparently been an overpayment, which amounts to nearly £250.

They state that the reason for the overpayment is because I receive 'Single Persons Discount in respect of your Council Tax'.

I will be calling them ASAP on Monday, but I'm having some difficultly understanding this letter. Does it mean that they were unaware that I was receiving 'Single Person's Discount', and so paid me the full amount of council tax. Or does it mean, they believe I am not entitled to 'Single Person's Discount'?

I live on my own with two children, so I'm not sure exactly what this is about.

Is there anyway to avoid paying this large sum? I provided them with all the correct documentation at the time of applying (April last year). I don't understand why it is my fault, when there is obviously some error on their behalf!