It's the inverted L-shaped patch. There's a hairline crack just visible round the edge of the patch and the dampness doesn't spread beyond the crack. The adjoining wall to the left is an internal wall and the dampness doesn't spread to that either. It doesn't get better or worse in the summer/winter or in dry/wet weather. The room is ventilated and warm.
Outside there's a chimney which was the flue for an old boiler that was removed years ago.
The internal opening was plastered over (no airbrick). There was a steel inspection plate on the outside base of the flue but that's been sealed as well now, as a ground-floor extension has been built round the lower part of the flue.
There are no obvious problems on the roof. The flashing and tiles look fine. The pointing looks sounds (looks like it might have been redone at some point). The guttering is clear and the cowling is solidly set in mortar (but at a slightly wonky angle).
I've just hacked the damp plaster off along the line of the hairline crack, and found that the damp area coincides exactly with brickwork while the dry area is breeze block.
I'm guessing that the chimney is the most likely cause of the damp, but I don't understand why it's not spreading and never varies. I've Googled the archive and opinion seems to be split on the cure for dampness due to disused chimneys. Some say ventilate at top and bottom (I could add an air brick above the flat roof level --- if there a metal liner in the flue that's connected to the cowling, would there be any through-flow of air or would I just create one opening into an otherwise sealed void?); some say that if the bottom is sealed the top should also be capped with a slate; some say that a cowling on the top is all that's needed.
Anyone got any ideas? Is the chimney the problem or is there something else I haven't considered? Advice much appreciated.