Strange flooring construction in upper flat

[Corazones]

Following leakage damage over a long period from a faulty toilet waste, in a purpose-built first floor flat about 20-25 years old, the soft, rotten flooring chipboard had to be removed.
I am puzzled by what lay below it. First an inadequate membrane of thin clear polythene, followed by very thick, non-metric 24" strips of plasterboard (which also disintegrated) lying on top of sound-proofing matting.
Is there a more modern replacement which can (or should) be used? If so, will it amount to the same thickness?

 

[ree]

It would help to see some pics?

Why "purpose-built"?

It was sometimes the practice to use sheets of plaster board as a sound proofer or even a fire precaution over floor joists in upper floors. The sheets of p/b are then covered with sheets of T&G c/board or ply.
In your case it seems that the joists have been furred out (packed) with strips of p/b to help with sound proofing. Perhaps the whole flat is done this way.
Or, its possible that the bathroom FFL was, for some reason, raised this way.
The plastic sheeting was presumably a kind of water proofing membrane to retain any leakage in the upper flat floor.

 

[tony1851]

Under the regs for soundproofing (explained in Approved Document E) it is suggested that airborne sound penetration between floors can be
improved by adding mass to the floor, and a couple of layers of plasterboard is a cheap way of doing this. Not sure about the polythene, though.