As i understand it, your managers can in fact set out your hours (37.5) in any way shape or form and this could include weekends or Public Holidays! The latter would be compensated for with a day off in leiu.(AFC Para 13 Annual Leave & PH policy)
The fact that employers generally haven't strayed from 9-5 is that this would significantly increase the pay budget due to unsocial hours working.
BIFF, make sure you point this out to your Managers, the fact that where these prospective shifts cover unsocial hours, weekends or PH's, this will entitle you to pay enhancement. Can't remember the definition of unsocial hours and pay off the top of my head but it is in the AFC terms & conditions.
Topper