Once there was a distinction between full time employees, those employed part time and workers hired in from agencies. Over the last 15 years part and full time workers have gained significant parity and from 1 October 2011, agency workers will do so too.
The changes are coming in under the Agency Workers Regulations which is based on the European Agency Workers Directive.
In essence, agency workers will be entitled to equal treatment when compared to an employee when it comes to basic working and employment conditions.
The areas covered include working time, rest periods, night work, paid holidays and public holidays; pay; protection for pregnant women, children and young people; protection under discrimination legislation; and access to onsite facilities.
Considering that it's easy for an individual to make a claim, you would be well served in understanding the detail and making changes to comply.