Can I take them to a tribunal?

I worked in a restaurant on new year's eve (I've been working there since 30th January 2010) and the tips from that night were put aside to be shared out at a later date. But, my manager text me yesterday saying all staff had to come in today to clean the restaurant and take down the Christmas decorations, and if we didn't we'd lose our NYE tips. I text back saying I couldn't because I had an exam today at 2pm (final year of uni so pretty important). He later text me saying he is going to share my portion of tips with two people who didn't work on NYE (because they had prior engagements, even though it was supposedly compulsory to work that night) but came in to tidy up today. Is this allowed? Can I take the management to a tribunal? It is not a chain restaurant.