So the latest, opened ceiling and noticed there was lot of play in the ply around the drain where the sheets met.
I put in timber to support the drain and also support the ply sheet joint. So no more movement from below. However although upstairs the drain no longer moves when I apply pressure the 4 tiles around the drain have a little movement when weight is applied.
I'm not really sure if possibly the movement has caused the addy under these tiles to blow.