it winds me up how cheap some people can be. that's a pricey house theyve got there and they want to save £3 a month of their window cleaner?
I'd remind them of the service they are getting and say no as you charge a fair price for you work. I would charge more for that house in my area.
make them feel cheap for asking, remind them everything you do and how reliable you are etc and your insurance etc.
you may loose them, you may not loose them, but if you cut the price it will wind you up everytime you do it.