Have some water in both the basin and bath then block the bathroom basin, and plunge the bath.
Agree with this. If you have two plungers and people, get one over the basin plughole and hold it down so the plunging doesn't force off the plug.
Cheers @hoefla and @rj will give it a go!
Edit update: it worked, woohoo! Thanks again.
@hoefla started
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Have some water in both the basin and bath then block the bathroom basin, and plunge the bath.