But Scottish, Scottish, Scottish.
According to you your customers DID pay up.
In the OP's they didn't and said they wouldn't.
You're comparing Apples with Oranges.
Okay, well just go and quote for his new house clean and add a Pound to what the clean would be. If he says its to expensive then walk away.
I would chap the old house and explain that I cleaned it but didn't realise he had moved out. If you want me to continue then the clean is a house warming present and we can start afresh next month. Might end up with 2 cleans per month and he is still going to pay for his old house clean but doesn't realise it. Then everyone a winner because he thinks he doesn't need to pay it.
In the end, I went out yesterday and did a full day on the glass and made nearly 4 ton so losing a tenner here or there makes no difference to me. I see it as part and parcel of the job. I only take Cash or Bacs so have no third party to pay.
Another thing I'm getting now is annual solar panel cleans even from none windy customers. Maybe they have realised it makes them more efficient.