Good letter, and a good specimen letter to use if you ever have something similar. However I would have thought it only worth the effort if you can actually get the customer back. It's not an argument you can win, and I believe you are smart enough not to worry about having the last word on a subject.
Personaly I would see it through to end, phone up, preferably talking to the bloke, say this is the window cleaner, I sent you a letter and don't want to fall out with you but I did what I thought was a very diligent job, and I would very much like to get to the bottom of what the problem was.
In some instances people do dump on the window cleaner, maybe the same sort of people who are rude and demanding of waiters and waitresses and then leave miniscule tips. Sometimes you just have to take it.