Hi Ian,
I can forsee problems in the 'wfp change over'. One of the guys needs a quick sale as he's got a new job lined up for next week. He has told all of his customers that he is selling up and they are just "happy there windows will still be cleaned". However if I turn up with a pole splashing water everywere I doubt they will like it.
I have bought wfp work in the past and was very please with how well it changed hands but I'm quite unsure about trad work.
Whats your views Ian?
Pro
you can do one of a few things, knock on each door indroduce yourself to them and say they way you clean, get the guy to give a letter to each of the customers saying that he has stopped you will be cleaning and you use wfp (saves loads of time) and get them to call if they need to ask questions.
Or just pole all the windows and explain if needbe.
I personally would ask the guy to give them a letter explaining the change over and saying you clean with wfp and to call you will any questions (most will not call).
Next one down I would knock the doors as I was cleaning and give a quick explanation and say they get there frames cleaned free for 2 months people like free things (if some of the prices are low this also gives the chance to increase them by a pound or 2 if needbe down the line "a great tip I learnt by mistake").
hope this help m8, post any more questions and I will do my best to help

Ian