If they don't answer honestly and get annoyed with you when you clock off after your agreed hours, it's their tough shit.

I worked for two young guys once who'd never had proper jobs, much less ever managed anyone before, and they had the cheek to have a go at me for only working my hours. I told them it wasn't my company and walked off. They couldn't sack me for working my contracted hours, regardless of if other people were working more for free...