Simon, you got me thinking again....
I can see the 4" vertical pipe coming down from the 3rd floor in the corner of the 2nd floor vanity room which is directly adjacent to the 2nd floor shower room/CR

The 3rd floor shower room CR I'd directly above the 2nd floor shower room / CR
I think I'll cut away a couple of square feet of the ceiling and see where the pipes are. Where they come from and where they go. This could well be my answer.

Don't know why I didn't think of that before.

Right, that'll have to wait until my ever increasing paint job have finished........

Feeling more confident I can get this planned out now.