Every time you clean a customers property, you class the charge as payment, even if you have not been paid, so at the end of the tax year you add this figure up and you put this down in box 3.29. This is you income for the year, even though you have not been paid for all of it.
If you look down on your tax return you will see a list of different expenses, on the left hand coloum is what you have paid out and on the right is what has come back into the business.
In box number 3.41 you put the figure for what you have not been paid, to the right is box number 3.59 this is for any bad debts from the previous tax year that you have been paid for this year.