Dear Custy,
Pay up now or I will randomly choose to either:-
1. Stand outside your house with a placard saying "Mr. X may drive a Mercedes-Benz but he has owed the window cleaner £20 for the last three months!" - then when Mr X drives past with his eyes fixed on the road ahead, follow him to the golf club and stand outside that too.
Then on Monday morning I will stand outside your place of work with the same placard.
2. Egg your windows in a hissy fit of rage.
3. Call at 6 am on Sunday and loudly but politely ask for my money.
4. Return the dirt to your windows.
Kind regards,
........................................