Roger - don't let them intimidate you mate - this is how it should go:-
Yummy Mummy turns up etc. -
You go round and from your clipboard you hand her a typed up sheet explaining the system and how you leave it wet "to dry naturally to a clear finish". As she reads this you price it up and say:-
"Your windows haven't been cleaned for a while Mrs. X so my rate will have to reflect that" you point to your table of rates which show different rates depending on heaviness of soilage.
e.g £20 shows £30 for first clean/£40 for heavily soiled.
You tell her that for monthly cleans you will charge (say) £30 for first and then £20 monthly or £25 every other month - which would she prefer?
Then when she moans you have taken less than an hour for your £30 you say as Shiner etc above have responded. If she's really awkward then take her money and tell her that upon reflection she does not fit the profile of customer that you require and good day!
I don't fancy you Rog
but you're a good looking guy with presence - use it and walk tall and act confident. Believe in yourself - you're a v. good w/c and it pains me to see you selling yourself short.
Sermon over!