Find a debt collection agency.
Say that you will sell their debt to them, and that you'll be paid either way-by them right now (IN CASH) or when you sell the debt. Say something like-"you should be aware that the agency will add £99 to the outstanding debt and have x years experience at small claims court level.
Try and make them think that you really don't want to pass the debt on but you've a business to run, kids to feed, mortgage to pay etc... So need to to keep afloat.
Try not to pull up in a 62 plate Ferrari if playing that card.
Say something like "the office is pushing forward on debts and I thought I'd go out of my way to save you the trouble-especially before Christmas"
Bear with me here- my Mrs gets her hair cut and pays a small fortune for it.
The guy that cuts her hair doesn't physically take her money. Someone on reception does. Sub-consciously what they are trying to do is associate the guy who makes her look pretty-therefore making her feel good doesn't take anything from her so she associates nothing negative (paying) with him.
If you can get your customers to associate you with the good of clean windows and the "office" with the negative of paying-you may even get to keep them as a customer (if you want them)
Just my 2bobs worth-but it's worked for me in the past. Only failed once in 10 years (lost £40)
www.laneswondowcleaning.com rutland