Juan. Just submit a small claims court application for the holiday pay amount quoting the 1984 employment act regarding "unequal treatment". You can't reward or punish employees doing the same job or guilty of the same misdemeanour unequally. You don't have to prove the validity of the holiday pay claim just that you have been treated differently from another employee. The court will be even more sympathetic because you are being victimised for not joining a union - all that closed shop stuff.
I'm going to do it once the pay is awarded, can't do anything before that.