Refusing to pay, disputing job but no communication! HELP.

Hi,
I did a job for someone and had all the costs agreed but when i asked for payment i was ignored. The relationship wasn't brilliant as she was quite difficult but I did what I was contracted to do and she agreed the costs. I chased the invoice after two weeks and was again ignored. After a month i called the customer who ranted at me stating that I'd left her with loads of problems. She hadn't contacted me at all during this time. I told her i'd come and see her to discuss but was told not to bother as she'd had someone else in to sort everything out and she would take off what she paid them from my final invoice. Since then she has refused to acknowledge my calls and emails/letters and has failed to pay. I haven't even any proof that there were any problems or if there were, what work was done! I have sent her a couple of letters asking her to pay and am about to send her another detailing the late interest and compensation that I intend to levy but am not sure how long I leave this before court action. I'd like to get my money as soon as possible as it's affecting my cash flow. She owes me several thousand pounds but not really sure of my rights. Can anyone offer any advice?