Hi Rabbit,
Try sending a registered letter outlining the situation and notify them as to the outstanding debt, mentioning further legal proceedings, if necessary. Thereafter, with no response, send a second registered letter (sent directly to top level within organisation) and if there is still no sign of payment, contact a debt collector regarding your debt. They will usually take a percentage of the debt recovered.
It's not the amount that's the issue here... it's the fact that they think they can get away with it that's really fustrating.