I see your point Paul. Maybe I should have used the word 'tactful'.
Whenever I have written to customers that I have had to cancel on, it's because I was actually taking too long to get back to them as I was too busy and couldn't keep up.
I did tell them this, so I was giving them the truth.
Any bad payers or messers don't get the courtesy of a letter. I bin them without telling them. Maybe that is impolite, but then I can't waste any more time on them than I already have, and they will know the reason in many cases anyway!
Like yourself, I have weeded out almost all of the messers, and I am now left with a generally excellent clientele.
John.