The Solution: The quoins from a sheet of Superquick stone paper. Cut out and stuck around the corners.
What you can't quite see is the bench seat against the back wall. The instructions recommend building this in-situ after the walls are folded and stuck, along with the window units... bonkers. Do it before the front walls are fixed and then you are working in an open fronted box and not poking around through the doorway like a gynaecologist. The addition of the quoins is time consuming , but changes the whole appearance. They're adjusted to fit the windows etc. and not used quite as intended. I also used some of the capping from the sheet to match the step to the quoins colour wise. Here the 'inner roof' has been added and at this point I used a 2B pencil to lightly shade the brickwork followed up with a bit of finger rubbing.