I have a Victorian-Edwardian terrace house with lath-and-plaster
ceilings. The ceilings have cracks in them ; what a surprise!
However, in one corner of the dining room (near the back window), the
cracks have brown stains running along them - perhaps half an inch
wide. I've just repainted the ceiling and the stains have come
straight through the paint.

Any idea what the stains are, and how I get rid of them?

Notes:
- The paint was Farrow and Ball casein distemper, which is close to
being the least impermeable paint known to man.
- There is no sign of damp in the room above, or on the external wall.
The ceiling doesn't feel damp.
- There is no down pipe in the vicinity
- There are one or two places where the vertical mortar lines are
missing (any one got a circular tuit going spare?)