Hi Dave, When I started building my round I used to print off batches of flyers explaing my service, and at the bottom I would give a date that I would be calling back, usually within 3 days, and an approx time, usually early evening (if your good looking like me, then try to call round before the husbands get home from work!!)
I would turn up looking very smart in your company uniform, look confident and friendly, and say something like, ' hello, I posted my leaflet through your door a few days ago and wondered if you required a window cleaner?'
Don't use the line about have they already got a regular window cleaner as it gives them an easy get out, and is not very positive in my view!!