I think you should put a written complaint into your employer, explaining exactly what you are owed and giving them a set time frame to pay it (say 7 days). Then if they don't pay up, I'd do what Flaminsam said and look into a tribunal hearing. Apparently there's also a 3 month time limit on a tribunal hearing so I wouldn't hang about! Failing all that, just get a summons issued to take them to small claims court, that should make them panic enough to pay up lol ;)