As always a wise and considered post Stu !!
basically i would do the same - i would ring up and agree a time i could pop round and explain the system and the stain - that way you can even give them a cloth and they can scrub it themselves - say you can restore the stain at £XX
i've had several custy's that take a while to understand wfp - so altho it's a pain i explain it to them at least twice - if you get
lazy here with new customers it leads to this sort of problem.
having said that - if they are looking to get out of having the service then it's any excuse they can think of - from that waters rotting my frames ( upvc at that )
to i don't like the dripping noise or i think you clean them to quickly....
only you know if you have cleaned the frames and whether they are mucking you about - but i'd say try and sort it face to face
many a time i've proved a customer wrong and has led to stronger customer relations with recamendations and extra work like conny roofs or gutter clears
Darran