Try the polite route first, i have a fantastic
relationship with one of my local letting agents.
i do all their cleaning, no quibble on price, always paid
within the 30 days, they occasionally deliver keys so
i don't need to fetch them. They also pass tenants
and landlords on to me that prefer to arrange things
themselves, and
6 or 7 seven years ago they late paid on an invoice.
The wife rang and politly asked if there was a problem
paying, shortly after the cheque arrived. since then they
have been a joy to work with.
Just remember due to the economy all sorts of businesses
are suffering, try to give them the benefit of the doubt,
although i think it has slid for a bit too long now.