I also find the same few are slow to send cheques.
I have payment required within 7 days in big red lettering on my slips - makes no bloody difference.
Have found offering bank transfer payment has helped, although I am usually running at upwards of 1K owing.
Pretty similar to my own experiences.
It's quite a while since less than £1k was outstanding and it can occasionally nudge £2k. However, much of this is from several cleaning companies who I subcontract from. Without that work it would normally be £500 to £1k. The difference is that over the past year, I have noticed an increase in slow payments from domestic customers. They pay up fine when I see them but some seem to have a block when it comes to envelope/stamp/cheque. Offering bank transfer has helped with some of these customers but so far, a lot haven't taken this up. In fact, the ones who have taken this up were mostly good, reliable payers already. I think I may need to be a bit pushier about this but there is always the concern about losing customers. Perhaps I need to replace them first in case they cancel when I start pressing a bit for payment.