Began to think I was in Germany!
I'll get straight to the point. The "basic" room is exactly as stated. Bare walls, no window (a skylight 8ft up which opened about half a foot); we felt like we were in the basement, even though we were on the third floor! Two single beds pushed together to make a "double" (they were comfortable at least, as long as you like a firm bed). No telephone!!!!! The bathroom was small but modern.
Now for the weirdness. All instructions in the hotel, from checking-in to logging on to the wifi, are in GERMAN first. They offer no explanation for this (and the wifi page doesn't even offer an english translation!) but after some digging I figured out that this hotel is owned by Germans. Presumably they advertise heavily in their home country, which is fine, but at the very least make German the second language, not the first.
All in all our room was ok for what we used it for (a place to sleep while we explored the city, nothing more) but £125 a night is a steep price to pay for what could easily double as a private hostel room. If we'd been staying in Edinburgh any longer we would've found alternative lodging.