UK ONLY. Can a debt still be recovered if I have already agreed a payment plan?

At the beginning of the year, I signed a direct debit agreement to pay off my rent arrears in monthly instalments after I was told that the debt was to be handed over to a debt collection agency. I received a letter from the agency, but the housing ociation with which I had signed the DD agreement told me that the agency would be called off once my first monthly payment was received, and I did not hear from the debt collection agency again. That was until this week, when I received a letter from the agency saying that I would be taken to court if the debt was not paid within 7 days. Unfortunately, the woman who organised my payment plan at the housing ociation is currently on leave, so I cannot get hold of her. Strangely, the agency are requesting from me the full amount of the debt, without deductions for what I have already paid off. What I need to know is this: if they do take me to court, will the case be thrown out due to the fact that I already have a payment plan in place? This is bearing in mind that the only thing I was given to sign was a direct debit agreement, and nothing with words to the effect of "We will not sue you in future" was signed. And if that is not the case, can it still be thrown out on the grounds that he amount is incorrect, due to my previous repayments?