Don't bother phoning, you'll get fobbed off again.
Go around there and be as nonthreatening as possible, but be firm. This guy is waiting for the inevitable to happen, and once he goes bust you'll NEVER see the money.
Ask him what he suggests for payment, explain you're a very small company and this is having a serious impact on your business. His money problems and his partner leaving aren't YOUR problems. If there are other staff there then that indicates he has the money to pay them!! Does he have a car? Then he has money for fuel.
When was the work done by you? How late is the payment?
Court action takes too long, I've been in this situation, on one occasion I sat in reception until I was paid, took about half an hour.
Don't get fobbed off with excuses, try and get a cheque out of him then take it straight to the bank.