You're entitled to 5.6 times times whatever you work in a normal working week. If you work the same number of hours per day then it's easy to calculate an answer in days (4 x 5.6 = 22.4) but it seems that you may work a different number of hours on different days so the calculation must be done in hours. (21 x 5.6 = 117.6).
You next need to work out how many hours you need to deduct from that for 'fixed' holidays (e.g. when the workplace is closed for public holidays on days that you'd normally be working) to calculate what you're remaining entitlement is.
If you want the detailed calculations done for you we'll need to know how the 21 hours are distributed across a week (e.g. 7 on Monday, 6 on Wednesday and 4 each on Thursdays and Saturdays) and whether your employer closes the business on all public holidays or on just some of them (such as Christmas Day and Boxing Day).
That's assuming the statutory minimum of 20 days plus 8 banks for a full timer, Chris - many employers give more than this, plus extra for 'long-service' etc. I agree I should also have asked for the working hours pattern for completeness.
Since the questioner was consulting the DirectGov website, Dave, I assume that the enquiry relates to the statutory minimum number of days (which, incidentally, is '5.6 times the number of days worked in a week, capped at 28 days, and not '20 days plus 8 bank holidays' since public holidays have no status whatsoever in holiday legislation).
Yes Chris - agreed that Public Holidays have no status in legislation - but as 'the bloke what works out the holidays' in several of my previous jobs, I can only say that people just do not understand this and they need to be considered/explained very patiently ... just lumping them in as part of the 'statutory minimum' may be technically correct, but it doesn't help people understand.
The concept that part timers have to put aside some of their holiday hours for 'Bank Holidays' when they would be otherwise be at work is a bugger to get across.
As Chris has said, you are entitled to 117.6 hours in total.
Each day you have off you must deduct the hours you would have worked on that day from the total - including any 'Bank Holidays' when you are not working.
So if you have a Monday off in week 1 you have to deduct 5 hours from your total allowance for the year, a Friday in week 2 would be 6 hours deducted etc etc.
It averages out to (approximately) 22 or 23 days off each year (including any Bank Hols) - but you need to keep an accurate record of the hours taken to be sure that you are getting the correct amount of holiday.