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?

What do you mean by *capped*? Chances are that it has a Drago-type air
admittance valve at the top. This lets air *in* to prevent a vacuum from
developing in the stack - but lets nothing out.

I presume you have another stack which is much higher, and vented at the
top?

Sorry, yes, you're right. It's not capped - there's a one-way air valve at
the top.

Now I think about it, there is a soil stack outside which runs to the top of
the building, despite nothing actually going into it at that level.

This used to have an even higher (and rather silly looking) pipe into the
top of it, going beyond the height of the roof. I know this from a photo of
the building when it was first renovated. This high pipe is no longer
there - I presume it got blown down at some point.