That's pretty crappy of them.
I understand your frustration. I was owed money from a magazine for a year before they paid me. Ah, the life of a freelancer.
You can't trust those Brits! Zembla magazine still owes me for three of the six crosswords I wrote for them. (Fortunately, they were paying me very well for them, so even with the payments averaged out I was compensated fairly reasonably -- but still.)