Never needed it because all I do is post a card and they either pay or they don't. After 2 cleans and no payment I post a card with the balance on it but don't do the clean. Usually they have just forgot and pay up and I clean the next month. Did like DD but now I'm not too sure about it when my van went down. It means you have a duty to clean or send the money back.
In the end, I like posting a card for payment so I got another 5,000 cards made up a bigger size so they can see it. My new size is A6 which is 105mm by 148mm and mostly clear on front so I can write on it, they cost about 3p a card.