For pretty much as long as I’ve lived in this house, I’ve had a theory about the boxed-in lump at the top of one of the bathroom walls; that it had been added to either:
a) as a vent from a bricked-up chimney; or
b) to disguise a reinforcing beam

I’d been hoping that the former was the case, since I could then remove it. But, on making a small hole, I found it’s true purpose. It disguises a rather large, unattractive pipe. So, sadly it must remain in place.