The old rule was that your statutory annual holiday entitlement was calculated by multiplying the number of days you worked each week by 4. (So, if you work a 5 day week, you'd have be entitled to 20 days holiday per year). Those 20 days included any days during which the firm closed for business (such as public holidays).
Under the new rules, the calculation is now 4.8 x the number of days worked each week. (i.e. for an employee who works 5 days per week, the total rises to 24 days, including 'works closure' days, such as public holidays. So it would appear that you're entitled to at least one extra day per year. (Remember that if Christmas Day and/or Boxing Day falls on a day when you wouldn't normally work, such as a Saturday or Sunday, you'll have to be given additional time off during the rest of the year so as to ensure that you still receive the statutory minimum entitlement.
From 1 April 2009, the multiplier increases from 4.8 to 5.6. (So an employee working 5 days per week will be entitled to a total of 28 days holiday per year).
http://www.direct.gov.uk/en/Employment/Employe es/WorkingHoursAndTimeOff/DG_10029788
Chris