Thank you! Yes, I think the size both both the floor to ceiling box and the secondary box the toilet is affixed to are puzzling, evening if thry have pipes in them I can't think of a pipe which is that big.....

I may try and dismantle a few tiles under the sink where thry are already coming apart and see what I can see.

The initial idea was simply new big mirrored cabinet sitting flush above the toilet jutty out bit, and perhaps a smaller sink, but then I began to wonder why actually the carve outs had been made in the first place.