Lee,
Take it as a lesson, if you buy a round then make sure everyone is aware that you are taking over, introduce yourself to them before your first clean. If they are not in leave them a typed letter with a note at the bottom telling them to let you know if they do not want you to clean their windows, othewise you will assume that they want to continue having their windows cleaned.
Make it a priority to get full details of all your customers, names, full address, postcodes and phone numbers so that if you do have any problems you can make claims against a named person and not just an address.
We are not in the bob a job game anymore, this is serious business and customers need to be aware that if they receive a service they need to pay for it.
In the past I have threatened a customer with court action over £18.00, they paid up and apologised.........then I dropped them.