Carl
I think your customer objected because you just added £1, too them it could come across as you wanting more money for the same amount of work.
I don't know your prices but when any of my customers require 2 monthly cleans, I always increase the price by 50% and then explain that it is because the windows require more work and effort than a 4 weekly clean.
I also point out that per month they are still saving money, but you will get some who don't see this, and these I dump.