Holiday Pay for Term-time employees

Hello All

I'm struggling to understand the right approach to holiday pay for our term-time employees.

They are paid for their term time weeks and annual leave weeks, spread out in equal monthly installments across the year.

Many of them do overtime when required on regular working days, and extra overtime during the school holidays when we run a holiday club.

Do I now calculate holiday pay on top of that holiday club overtime at 12.07%? What about the overtime worked on regular working days? 

I understand that an employee shouldn't be worse off financially during their annual leave weeks. 

If anyone can provide a simple explanation or point me towards a helpful guide or template calculator, I would be most grateful. 

Many thanks