You are always going to get one or two people like that, I have had a couple before but just dropped them. Its not worth wasting your time. But remember even if its not the law, if anyone get word that it was you, your reputation could be damaged and it could cost you more. It is the law that you should be paid for your work though, but unfortunately it cost you more money every time you try and chase it up.
My advice it forget it and go and get another two customers that will treat you better, and lets face it, they will soon run out of window cleaners to flease and will be pretty diry soon enough anyway.
I once had another window cleaner warn me that one of my customers owed a bit of money to another window cleaner, and low and behold she was a pain in the beep. I never let her away with anything, and reminded her that she would not be done again untill she paid up for her last booking. Once it had been about 4 months since I did her and she asked me to do them again, and ask for her ballance be settled and got it, I even reqouted and added another couple of pound, but in the end I never went back when I got my money, not worth the hassel.
If anything I would do that, let the other cleaners know what they are like and to be careful, in turn damaging her reputation instead of yours, becuase thats what she deserves, no-one to trust her with a thing.