This site may help.
http://www.direct.gov.uk/en/Employment/Employe es/WorkingHoursAndTimeOff/DG_10029788
I don't understand your calculation or your employer's calculation. Your calculations don't make sense- are you mixing up hours and days?
Let's start at the beginning. Are you saying a full time employee (working how many hours a week?) would get 4 weeks (20 days) holiday a year?
Presumably that doesn't include bank holidays (since if it did it would be less than the statutory minimum). Adding 8 bank holidays would make that 28 days holiday a year which is the statutory minimum (5.6 weeks) from April 2009 so could be correct for a full time employee in your company.
On this basis you are entitled to 5.6 weeks at your normal working week which is 28 hours. So in terms of hours that's 5.6 x 28= 156.8 hours per year including bank holidays. Or it could be 5.6 weeks but that gets complicated depending on how your employer treats bank holidays for you.