Finally the rabbit hole mystery hole solved! Alex, the smallest builder there is apparently the one that always draws the short straw and gets sent up all sorts of small gaps in their job. He squirrels in and opens up the boards sealing the dormer window – the rest climb through the dormer.
I was lucky enough to get in early enough and climb in too. Lots of progress on the inside. Rooms being planned out for the walls, sockets, radiators etc.