It is like Newchodge said
Basically start with ignoring bank holidays - all employees get 5.6 weeks holiday per year. A week/day in this case is calculated by averaging their past hours to work out an average "day"
Then you can contractually say that some of that covers Bank Holidays (or set shut downs in summer or over xmas) but the basic facts remain the same - the worker accrues 5.6 weeks holiday pay over their leave year, your contract may then dictate when that is taken (unless you have been stupid and inadvertently given bank hols as extra hols on top of statutory by poor wording but then that is your fault)