Proper annoys me when people behave like that, no reason for it at all apart from they think they can rip someone off and get away with it.
I guess she's never thought of sending you a cheque to cover it...... No excuses really. We can accept payment on line for those with a computer and half a brain, saves running about.
I'd send her a snotty letter giving her 28 day's to settle the outstanding debt before you hand over the collection to an agent and advise her that there will be a collection fee of £120 charged for late payments as per your company policy which she verbally agreed to

Probably wont work if she's a crafty cow but if she's just a lazy mare it might give her the kick she needs. Plus your not going back there so it's only the cost of a stamp.
If it don't work at least you tried mate, Karma might bite her at some point.