The Agency Workers Regulations say that after 12 weeks on an assignment in the same role for the same hirer, temporary agency workers are entitled to the same pay, holidays and working time as a full-time equivalent doing the same job.
The temporary agency workers at your workplace may have signed agreements known as 'Swedish derogation' contracts. This is a loophole in the legislation which has been used by some employers to avoid the obligation to pay equal pay (but not hours or holidays). Not for much longer.
Following union campaigning, including a formal complaint brought by the TUC to the European Commission, the government has announced that the Swedish derogation loophole will be closed from April 2020. From this point, employers that try to escape their equal pay obligation by using this mechanism will be breaking the law. Speak to your union rep to find out more.