Staff are on a zero hour contract, when signing the contract there is a section (for those who read it) which states: on some occasions you will be required to work more than 48 hours a week to meet the needs of the company, by signing this contract you adhere to these requirements and subject yourself to long hours. (not word for word but it's similar to that. If you cannot adhere to the lengthier hours, before signing the contract you need to contact HR and discuss which may be refereed to your line manager
Staff aren't paid overtime either, they have the standard rate even when they are working unsociable hours such as Fright Nights. Hope this helps