Basement stairwell tanking concern

Hi,
This isn't DIY, but you guys were helpful with my last question about my basement and so I'm hoping you can help again. Sorry for the slightly long post, but detail might be relevant.
Builders are currently tanking and converting my basement and have installed a waterproof membrane, drain and well/pump in the main area of the basement.
However they've said that they've had to change plan on the stairwell which leads to the basement, due to the restricted width leading down there (there are brick walls on either side of the stairwell). Instead of hacking back the plasterboard which is dot-and-dabbed to the walls either side, fitting the membrane, battening and replasterboarding as originally planned, they're applying a Visqueen liner directly against the existing plasterboard, over which they're fitting ply and painting. This Visqueen layer will be taped to another sheet of the stuff which will run down the stairs and which will be covered by thicker marine ply.
Apparently, the walls either side aren't straight (or square with the stairwell) and to fit the membrane would require the wall to be rebuilt, which would cost extra. If I'm mistaken then I'm sorry, but that sounds like total bollocks to me. Wherever else the wall hasn't been straight, they've compensated with the battening, which they've fitted ON TOP of the membrane, before affixing the plasterboard. I don't see what bearing the straightness of the walls have (within reason) on whether the tanking system works or not and, although the stairwell certainly isn't dead square with the walls (carpet had to be fitted in two step sections) it certainly isn't ridiculous.
The stairwell footing is concrete slabs resting directly on the ground (soil etc beneath the house) and, when the brick walls either side were first plastered (dot+dab plasterboard skimmed over) and carpet laid over the concrete steps, I very quickly saw signs of mould and damp coming in on the plasterwork, near the base. At first this was treated by stripping off the affected plasterboard, scraping the bricks and concrete steps clean of mould, applying mould killer, leaving for a week and then applying a rubber waterproofing solution, before replastering. The mould hasn't returned yet, but I suspect it will if not fixed up properly, albeit it might take a longer period of time. Certainly the paint-on rubber solution is already full of holes and the current solution doesn't strike me as "proper".
My concern is that these tanking systems are built specifically to allow trapped moisture to drain away at the bottom or evaporate through the top. Visqueen sandwiched between ply and the plastered wall (fixed as described above), taped to another sheet of Visqueen running down the stairs, in turn boxed in with ply, doesn't sound convincing.
Am I being unfair? Is this an acceptable solution when required?
Thanks in advance.
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Err - plasterboard pretty much disintegrates when it's wet. To the extent that it won't support its own weight. If it is wet enough to actually require tanking, it may only last a short time, before sagging down to the floor.
<snip>

Battening non-straight walls is annoying.
Is it a fixed price contract or something?
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wrote:

long
installed
basement.
which
(there
back
fitting
they're
over
It was never *that* wet - it was just damp/mouldy enough for it to show through after only a couple of weeks at various points near to the ground, when the job was first done inadequately (without tanking). But it was holding up happily enough after over six months. In fact the newly installed well where the moisture from the drains collects has remained dry after a few days which, I gather, is unusual. The house is geographically quite high up though, so I presume there isn't as much groundwater about.

straightness
not
Yes, it's a fixed price job.
The question is, is their method for tanking this area acceptable practice? Is it OK?
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