I think you'll find that they're deliberately holding back payment untill the very last moment and trying to get away without paying for as long as possible thinking that a small tradesman won't have the "balls" to take it further.
My only outstanding debt at the moment is for a judge'e house. After asking in the area, I've found out that he does this to everyone as they're to scared to take on a judge in court.
On the other hand he dare not have proceedings against him.
This is true. I threatened (in writing) a Solicitor with a small claims action. I got the printed forms and filled them out. I photocopied them with a letter basically saying that if I hadn't received a cheque for (about £525.00) within seven days that the claim would go ahead with no further notice AND that I would contact the Law Society.
He paid up.
(I used to sell office equipment to Solicitors etc and one I got on well with advised me what to do saying Solicitors dare not have any court action against them as they would get into big trouble with the Law Society. (Solicitors' professional body.)