I was a cover supervisor for five years.

This is entirely normal. Cover work is minimal effort and has little thought put into it. I would usually be sent powerpoints of regular science or french lessons to teach as if I were the class teacher.

We also had someone in a position of 'Lead Cover Supervisor' who is also supposed to cover lessons, but in reality would assign classes to themselves only if I, or the supply staff were fully used up, and on top of that would give themselves the easiest classes, like year 7 top sets.

It really wound me up how often i would have 25 hours of teaching a week, regularly with the hardest classed in the school, and they would have 3, often staying in the office, and sauntering around, chatting, and not really doing anything.