left half-way through and pay still outstanding, any advice?

I had an internship in the financial services recently and though i was initially super-excited about it, my line manager was a total b!tch, and finally after several weeks of torture I decided that I could not bear it anymore and opted out. Even though i was told on the departure day that i would be paid up to my last day of work, that did not happen. Do you guys think it's worth pursuing the matter or just forget it? if yes, how would you go about it? thanks. xx