How does one sue?!

Hi, I'm a new member here and I admit I have no clue about legal steps about a certain issue. I'm a designer and I work as a freelancer. Recently I did an illustration based project with a publishing company which has its headoffice in the UK. The contract stated my payment in 3 installments, out of which I have received the first one. The work is done and I'm yet to receive any further payment. Moreover, like an idiot gullible soul, I had meanwhile given my copy of contract to the person who was mediating on behalf of the company! Now my question is: Is there any way I can force them to pay me the remaining amount?! Or I better forget about the whole deal and make a few angry phone calls at the most! Thanks p.s: sorry if I have posted this in the wrong sub forum.