Update: (or "Christ, the f*&K up gets worse")
We ordered a wall hung toilet.
Having cut a hole in the skimmed plasterboard I was reminded that there was another pipe (behind that insulation in the picture) in the way of mounting the Grohe frame. Feck it.
So, tomorrow, I pick up another bunch of bends and create the latest water flume (not open to the public for rides!!) to re-route that downpipe, from the cloakroom above, towards the corner (to the left) so the frame will fit in to allow the wall hung toilet to empty into the visible socket on the right in the picture.
Also, the brown pipe will be replaced by a Y junction, and so the white waste pipe and black boss will also have to be refitted.
Time for a🍺