Chimney breast removed from downstairs room only, now ceiling is leaking.

Chimneys are a bit difficult to diagnose sometimes, can be a bit of trial and error. From what you say, because the water is not showing on the first floor it is unlikely to be chimney flashings. I would start by checking if the water can simply enter the top of the chimney - are the pots fitted with pepper pot caps, what is the condition of the chimney ie has the flaunching deteriorated (Mortared bit at the head holding the pots on). Check the chimney pointing, to see if that needs attention. If the chimney has been capped of it could be condensation, in this case you may need vents in the bedroom or airbricks in the stack itself. I have had in the past where i have checked all these and been left with the possibility of porous brickwork, allowing water penetration, this is the most difficult to resolve without rebuilding and incorporating a DPM, I have had some success with water repellent coatings.

without looking at the chimney is is not possible to say exactly where it is leakin but it sounds like either the flaunching or the chimney pot if it is not capped, the bottom of the chimney breast should be vented, call post the job in roofer/chimney s and get a local expert round to check out exactly where the leak is and how much to repair.

you will need to cap off the chimney pot with a vented cowl and fit a vent into the chimney breast in the bedroom. this will stop rain getting in and allow any condensation to escape. get a builder round.