On the far side of that full height riser is the bathroom

On the far side of that full height riser is the bathroom
By Phil Murphy

The riser serves more than one flat at every level, undivided.

Over fifty years, the DIY enthusiasts (and plumbers) that changed their bathroom ceramics are likely to not have understood the need to fire stop any work. The likelihood of plastic pipes melting providing an unprotected hole through the barrier are considerable on every level.

Not being able to easily see where it goes through the wall (without accessing the riser itself)gives rise to the problems associated with fires inside cavities. Only a Type 4 FRA would unveil whether there is any fire stopping in place.