From 1st October 2011 new legislation was introduce regarding the rights of Agency Workers.
If you employee temporary agency workers you will need to read the new legislation found here:
The legislation means that after 12 weeks of qualifying work they will be entitled to the same rights as a permanent employee e.g. Annual Leave, rest breaks, key elements of pay etc. A qualifying week is any work within a 7 day period, so for instance, if an employee only works 1 hour per week this is still classed as a qualifying week. There must be a break of 6 weeks between assignments in order to reset the 12 week period (or a break of over 28 weeks if the worker is sick or on strike or jury service).