The thing is , we know we can do a £10 front x 10 of them faster then prob that house and if one of the fronts mess us about it aint so bad , but when you do larger houses if they become messers, its a big problem, personally i wouldnt give a customer £250 worth of credit i used to in the past, not anymore not worth the agro,
simply say that you are selling them your services and will do your best however you can not please everyone and due to a few bad expiriences you now take payment upfront paying upfront doent void them to a reclean of certain windows should it require it but you are the final decision maker in the standard of the clean this is in order to protect yourself from the OCD customers.
after the first 2x they pay you upfront then they will be given credit the least i would go is 50% then 50% (and thats only if i had a very good vibe), if they cant understand that you are going to spend most the day there and your rules considering its the first time you have worked for them, and refuse to pay a certain amount upfront i would simply walk.
back to your £15 all rounders gate opened ,easy life... get ripped off for £15 compared to half a day or a full days work inside cleans too no thanks