If imposed rostering is already the issue, just impose another one. It can't be any worse.
I don't entirely get why actual rosters have to be agreed, and don't see why that sort of micromanagement by Unions is tolerated. I do get that it isn't acceptable just to change peoples' shift times, but if you're on shift you do what your employer wants between those times. For instance today I've been asked to do some data checking instead of my normal work, and I'm doing it, because I'm being paid to do it so I do what I'm told.
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That seems to imply that they think people are "pulling sickies".
If it is predominantly caused by staff sickness, why is it always Fridays and Saturdays worse hit than Monday to Thursday?