Believe it or not this soil branch (connects the toilet to the soil stack) was installed by the original builder / developer. It was originally concealed by the large box out (where now exposed block work is). The customer had complained that the toilet didn’t seem to flush very effectively, I’m sure most people can see that the flushed toilet water (and waste matter) wont flow uphill, so I haven’t a clue what the plumber / builder was thinking when this was installed. This wa on a well known development in Burgess Hill. I have heard that the builder had cut costs by getting labourers to install the plastic waste pipes instead of qualified plumbers – but I cant verify that.
Anyway, I removed the offending item (yuck) and installed an air admittance valve and a soil branch that sloped the right way.