I can't find the exact shower waste fitting on the web but it's a shallow chriomium plated plastic dome about 5" across with what looks like a 3 sausage-segment air vent holes in the top, about 2cms P.C.D., waste shower water is intended to drain away under & around it's perimeter. 'Cept it doesn't without that bit of persuasion. It seems it might be at least similar to something called a "Fastflow" waste.
The walk in shower replaced the existing standard bath, but the waste is perversely at the walk in end and the pipe had to be extended I'd say the run to the soil stack is a bit over 3 metres and the fall is less than the depth of the joists with one right angle bend. However with the domed chrome insert removed it drains OK. If anything has to be changed under the floor it can't be done, it's all sealed up :-(
I wasn't aware that these shower wastes were such a complicated affair with concentric cups and sleaves. Exactly what it is all supposed to do defeats me. Anybody come across anything like this before P.S.E. ?