Hi, welcome to Just Answer. I will help you with your question.
If nothing was agreed about repayment, then it would be considered a loan that is repayable on demand. As such, if she asked for the entire amount back immediately, she would be able to proceed to Court on that basis and demand the entire sum be repaid now.
However, if you agreed terms, such as payment of £250 per month, then she would have to stick to that.
But assuming the first is the case, and there is no agreement, then it's nothing something you would want to get to Court as she would be entitled to all of it immediately.
Unless you can argue that she agreed, impliedly if nothing else, to accept a lesser sum each month.