I partly agree with kempie in that it depends on what your contract says, how many hours you work compared to full time employees and how many holidays/public holidays the full timers get.
It would seem odd to me though if a Saturday holiday is treated the same as a Friday holiday. Just imagine if the company only allows her to take holidays on Saturdays- she would have far fewer hours off in a year pro rata than a full timer.
I think the bank /public holiday situation is a little less straightforward than Kempie suggests (though I accept I'm no expert). If full time staff get 8 public/bank holidays then I understood part time staff who work 50% hours (more likely to be women) could claim discrimination if they consistently get fewer than their fair share of bank holidays (eg because they work say Tuesday, Wed, Thurs).
So in fruitsalad's case the figure of 15 holidays includes bank hols so I'd like to know how many holidays/bank hols she get compared to full timers.
To sum up, fruitsalad, it would help if you could tell us what your contract says about holidays and bank hols, how many hours a week you work, how many hours a full timer works and what entitlement to bankholidays/holidays a full timer gets. Then I'm sure kempie/buildersmate/Ethel/Barmaid , etc could resolve this one