Hi Ed.
The statutory holiday entitlement is (for those who have a regular working pattern) 5.6 times the number of days worked each week. (So someone working a 5 day week is entitled to 28 days paid holiday per year. The entitlement for 2009 is actually only 27 days because the current rules didn't come into force until 1st April). The holiday entitlement includes all 'enforced' holidays, when the business is closed, such as Christmas Day and (where appropriate) other public holidays.
When someone has an irregular working pattern an equivalent holiday entitlement must be calculated, using any appropriate method. In the case of someone working 30 hours per week (but perhaps with a different number of hours on each day), the total statutory holiday entitlement must be 5.6 x 30 hours = 168 hours. (For 2009 the actual figure is 162 hours because the current regulations only applied from 1st April).
http://www.direct.gov...dholidays/DG_10029788
Chris