zero hour contract 
How does a zero hour contract work? how do you calculate holiday entitilement?
You have the right to take statutory paid holiday from work if you are a worker. This includes people who work full-time, part-time, agency workers and casual workers. Only people who are self-employed and a few other exceptions will not be entitled to statutory paid holiday
To work out how many days holiday you can take a year, you need to multiply 5.6 by the number of days you work in a week.
TBH zero hour contracts will not attract decent people or even help you retain them.
You better off guaranteeing a smaller number of hours per week and topping up with overtime if needed.