New regulations came into force in October 2011

New entitlements:
Day 1 rights for all agency workers: If you hire agency workers, you must ensure that they have they can access your facilities (such as canteen, childcare facilities, etc) and information on your job vacancies from the first day of their assignment.
After 12 weeks in the same job: These additional new equal treatment entitlements relate to pay and other basic working conditions (annual leave, overtime, etc) and come into effect after an agency worker completes a 12 week qualifying period in the same job with the same hirer. Pregnant agency workers will now be allowed to take paid time off for ante-natal appointments during an assignment.
It is not retrospective and for those agency workers already on assignment, the 12 week qualifying period will start from 1 October 2011. 

Full guidelines on the BIS website.