"Graeme" wrote in message
I'm just aiming for a perfectly flat floor. Without some sort of
filler/levelling compound, the ply will show the undulations that are in the
existing chipboard.


Here's what you do - get a circular saw, cut around the perimeter of
the bathroom floor, buy some WBP ply same thickness as the chip, cut
it to size and install in the hole you created. Job done.