As well as providing a statutory amount of paid leave the Working Time Regulations also provide that a worker should receive a week's pay for a week's leave, and that the week's pay should be calculated in line with the Employment Rights Act 1996 (the ERA).
Employees who are contracted to 20 hours per week can be said to have normal working hours, and so the amount of pay they should receive is that which they would get if they worked those normal hours (S221 ERA). In addition to this S234 of the ERA specifically provides that in the case where an employee with normal working hours does overtime which takes them above those hours then this is not counted towards the calculation for a week's pay. However, if the overtime is contractual (i.e. you explicitly require an employee to do x hours of overtime per week in their contract) then these would count towards the normal working hours.
If an employee has no normal working hours or can in the case of a 1 hour contract show that it does not represent expected workload then the method of calculation is different. For them, a week's pay would be calculated by taking the average of their last 12 weeks earnings (S224 ERA). If they have consistently worked 40 hour weeks then they would receive pay at that rate.
As far as demanding holiday pay on overtime hours - you can demand all you like but have no legal basis for doing so and so are entirely in the hands of your employer.
