I'm not sure any of the laws quite fit here although I like to think equal pay for equal work should apply. But equal pay only really applies if you can prove that its sex discrimination. so if those of one gender are paid more than the other in the same job.http://www.direct.gov.uk/en/Employment/Re
solvingWorkplaceDisputes/DiscriminationAtWork
/DG_10026665
Alternativley there's the equality act or discrimination act but they tend to apply to protect people being traeted differently on the grounds of race, age, gender etc.
The one that is closest I think is the one that protects agency workers and offers them full rights and equal treatment to the others after 12 weeks.
http://www.direct.gov...entagencies/DG_198917
The best thing to do is to make an appointment to see the most relevant person in the organisation eg HR, line manager, and propose a pay rise. Explain the inequality and make them aware of your disatisfaction witht he situation.
If that gets you nowhere then at least you've made a start and can then talk to the union rep or go back to HR and raise it as greivence.