Why do you need to offer an hourly salary? How about offering an annual salary for a set number of hours per year?
It's sort of the same thing but may help you to work out a set monthly pay, so that you are not constantly going back to the payroll company to change the pay figures - which may incur you additional payroll costs. Talk to the payroll company about what things they charge for and how quickly a change in working hours can be reflected in the pay. Ask them at what point in the month you have to tell them the hours that are being worked that month, so that the payslip can be produced for towards the middle-end of the month. Also you mention about weekly pay - do check with the payroll company about cost of doing weekly pay vs monthly, as they may charge more for processing more payroll runs.
The difficulty though with doing a set monthly pay, rather than paying for work actually done, is that you will have paid for work yet to be done, so you need to keep a good track of hours actually worked, so that if they leave, then when working out their final pay, that last month of pay may actually be quite low as it is just wrapping up what is remaining to be paid (or in a bad case owed to you).
Could you do a spreadsheet with the projected weekly working hours on it, along with the pay on a per-hour basis and a pay on a monthly basis? That may help show how much of a difference there is at varying times of the year.
Good Friday is only deductible from annual leave if it is actually a working day.
As they are not working fixed hours, you can't do holiday as days... it will need to be done in hours. So for each hour worked, they get a certain amount of holiday entitlement. I think you use the irregular working patterns method which is 12.07% of hours worked.
If they start before Christmas, they may not build up enough holiday to take off the time between Christmas and New Year (if you are not wanting them to work then), so you may need to let them take some advance holiday. So keeping track of holiday taken is vital, so you know at any given point if they have taken their entitlement, not taken enough, or taken too much. You can also give more holiday than you legally have to, so you could decide to give some hours of holiday in addition to what they accrue.