Part-time and agency workers
If you're a part-time worker, you're still entitled to 4.8 weeks holiday – 4.8 of your normal working weeks. For example, if you work two days a week you have the right to a minimum of 9.6 days' holiday (that is, 4.8 two-day weeks).
A part time worker should be treated no less favourably than an equivalent full-timer. This means that an employer who gives extra days off to full timers may have to give extra time off to part time workers as well.
If you're an agency worker, your agency must allow you to take your paid holidays.
http://www.direct.gov.uk/en/Employment/Employees/Timeoffandholidays/DG_10034642