But, a little disappointingly, it just appeared to be a rubble and wood strewn room. Only when I stuck a camera and flash through the hole did the reason for the walling-up become a little clearer. I discovered that the kitchen had had a coal hole, the door of which had later been blanked off....but the doorway and door were left in place and a wall put in to create an outside toilet.
The fact that the panel blocking the door looks like insulation board and has a modern footprint on it suggests a relatively recent development, but what I don't yet understand is when the coal-hole door was put there. It is right in the place where the kitchen chimney once was, which I'd assumed developers had removed just before I bought the house, to create more space in the kitchen.