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.
However, the temporary agency workers at your workplace may have signed agreements known as 'Swedish derogation' contracts. This is a loophole in the legislation which is being used by some employers to avoid the obligation to pay equal pay (but not hours or holidays). Speak to your union rep to find out the position.
The TUC has lodged a formal complaint with the European Commission about the UK’s failure to implement the Agency Workers Directive due to widespread adoption by employers of Swedish derogation-style contracts, and is campaigning for this loophole to be abolished.