I find it amusing that whenever debts come up there's a sudden rush of barrack-room lawyers appearing who fire off their opinions about the law.
Forget the law. Look at your time and quality of life. Do you really want to spend time on legal shenanigans that, with a following wind, hours of effort and a court case regarding the exact contract you have with your customer (taking previous case law and precedent into account) might get you twenty or thirty quid out of a customer?
No. Take reasonable steps (I set the limit at about 15 minutes total) to get the money out of people whose windows you actually cleaned and who didn't pay. Then dust yourself down, forget the money you didn't collect and move on to customers who want you to clean and who want to pay you. Don't spend time chasing them to court, spend the time with your family, watching TV, picking your backside or even looking for good customers.
Life really is too short.
Vin