Managed to unscrew u-bend, but as I rotate underneath sink the whole drain cover rotates. There’s what looks like something for a very wide screwdriver to stop that (for which only a chisel is wide enough), but it won’t undo.
That's not unusual for it to do that, even when installing them. Try putting two screwdrivers into the waste outlet sink side in different hole sections and hold the two screwdrivers as you unscrew the trap. This should lock the waste while you undo it.
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