The first thing you should do is check your contract of employment for the express conditions.
Holidays are a minimum of 5.6 for each day of the week worked per year, if you work 5 days a week multiply this by 5.6 and you will come to a minimum of 28 days per year (if you worked 6 days per week your minimum holiday entitlement is still 28 days) your contract may give you more. Strangely there is no compulsion to pay for overtime at all provided the average does not fall below the minimum wage and your holidays can include public holidays, though this is unusual , some professions such as teachers do not usually get paid for overtime as living with a wife and daughter who are both teachers I hear continuously .
Part time workers should also be treated no less favourably than full time workers so your holiday entitlement and pay must be pro rata to your full time colleagues.