Ring again, speak to someone else, I was over paid, I owed 1200 and they were fine about me paying it back over 15 months, if they get snotty again, tell them you have seeked advise from a solicitor, and if they dont allow you to pay it back over a time span yoou can manage, you will take things further, in other words, turn the table on them.
Just another thought, surely if they had not recieved your declaration your payments would have stopped.