My house is upside down - kitchen/lounge upstairs, bedrooms/bathrooms
downstairs. Roof is still on top of the house

In the en suite, I've got what I sugess is a soil stack pipe... except that
it's capped at around 1.5 metres in height. It requires its own plastboard
housing so it can be hidden away.

The toilet goes into it at the bottom, as does the sink and the shower
drain.

Anybody know the reasoning behidn it? I guess it's so that if there's a
blockage further down the drain system, there's space to backup? But,
equally, woudn't that backup head up the sink/shower drains and start to
overflow? What can I do about this? Is there a non-return value system?