The Medway Handyman wrote:
Weird today. En suite with WC, basin & shower. Boxed in soil pipe in
rear LH corner. Shower to the right of it, basin & then WC along LH
wall.
The shower waste 'gurgles' after a short while. Initially it drains
OK, then the tray starts to fill a little bit, then air bubbles come
up.
Running a tap at the same time as the shower transfers the 'gurgle' to
the WC, which initially causes the level in the pan to rise.


Water can only rise in the pan if there's water higher than it in the ****
stack, or, in rare cases, there's a vaccuum, in which case it'll need a air
inlet valve


Due to their position, I would expect the shower waste to be connected
to the soil pipe via a boss and the sink to be connected to the WC
soil pie - so the shower & sink waste/WC shouldn't be connected.

They won't be connected, but if you're running a tap and the crapper's
filling up, this is an indication that the stack's blocked and you are
effectively raising the level of the blockage


Can't see though, everything is boxed in & tiled over.

Plunged the shower vigorously, carefully poured some 'One Shot' in, no
difference.

I have a feeling it may be air?

Only accessible part of the waste plumbing would be the shower trap.


It's probably air.
If they want it fixing permanently, the boxing in will need breaking open
and an air inlet valve fitting, unless you can get one outside.
The stack can't already be open at the top or this would never happen