The first answer, above, may be the only way that your holiday pay (per day) can be worked out.
It should be remembered, however, that the normal way that a holiday allowance is calculated is not based upon the number of hours which are worked each week but the number of days each week over which these hours are spread. I.e. the formula states that the number of days paid holiday per year is 4 times the number of days worked in each week. So, if you were working 5 days per week, your total holiday entitlement would be 20 days.
If your '6 to 13 hours' is normally spread over two days per week, then your total holiday allowance will be just 8 days per year. If the days you work are Monday and Tuesday then (assuming your firm closes on all bank holidays), every year, Easter Monday, May Day, Spring Bank Holiday, and August Bank Holiday will take up 4 of those days. If you'd have been working at the start of this year, New Year's Day would have counted as one of your days hoilday entitlement. (This is because it fell on a Monday. It wouldn't have counted if it fell on a day when you didn't normally work). Also, this year, Christmas Day and Boxing Day fall on a Monday and Tuesday so they'd come out of your entitlement.
All of the previous paragraph means that, if you'd have started work at the beginning of the year, working on just Mondays and Tuesdays (irrespective of the number of hours which were spread over those two days) you'd only have 1 day of paid holiday entitlement (other than Bank Holidays) this year. The amount you got paid for that day (and for each of the bank holidays) would be calculated by the method given in the first answer.
I can do some more accurate calculations, if you wish, if you'll indicate how many days you normally work each week (and also which days they are).
Chris