The Agency Workers Regulations 2010 came into force on the 1st October 2011. These provide entitlement to agency workers who meet the qualifying period to the same basic working and employment conditions as employees directly recruited by the hirer. This of course includes pay.
If you are an agency worker and are unsure of your rights, you should seek legal advice at the earliest opportunity. If you are concerned or being dismissed or bringing action as an agency worker, you should note that any dismissal on the grounds that you have brought an action pursuant to the Agency Workers Regulations will be automatically unfair.