PoP wrote:
Just musing over whether bathrooms in flats are plumbed differently to
regular houses......

I my limited experience this sort of thing is standard for a conversion,
especially in London. At my current flat, the WC waste was actually on
a slight upward slope! Amazingly, it only blocked occasionally. The
same bloke must have done the flat below, because nearly two square
metres of the inner layer of the exterior wall was demolished to make a
small hole for a 110mm vent. This left several joists under a cupboard
unsupported in my flat, and it very nearly collapsed when I converted
that space into a kitchen...