I only do one clean now, chase late payers by statement and text.
The worst I can lose is one clean.
In the past my last attempt was to send a letter stating that I only clean the windows and at the end of the month the accountants would take over the control of the debt. The accountants would immediately pass the debt to their DCR agents, debt recovery agents who would be the next contact.
This contact would be a demand for approximately £110, purchase of debt, search fees, interest, administration etc, Failure to pay this would result in court action.
All bull but a final attempt and it worked in all but one case.