Kitchen sink overflowing into Bath/Bathroom sink?

[DOLL]

Looking for helpfull answers - Get a plumber is not helpfull !!

Water from kitchen sink is coming up the bath plug hole when its drained. This is also the case from the bathroom sink.

At first it would eventually drain off but now the bath is totally blocked. I have tried plunging and pumping to no avail.

Is Costic Soda or similar products a good idea or do i have to remove all of my bath panels and start checking ubends etc for blockages?
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Kitchens Bath