Surely the whole point is that you can only do part time work that is why you are applying for a part time job 
How are you supposed to arrange to work extra hours for someone's fortnight of holiday, or worse, someone's sudden sick leave?
I assume most people, like me, work part time to fit around children. But even those without children are likely to have other fixed arrangements when they are not working, could be anything.
It seems wierd to have a part time job but have to be available for full time hours? Am I missing something? Can you tell I'm getting fed up with looking for suitable work? 