I moved into the property (a 1840's semi detached cottage) in 2007 a few months after moving in I spotted some damp on the wall in the bedroom. We had a local chimney man in and he advised that the cowel we had fitted wasnt correct and that the flashings were poor. We had him replace the cowel and address the flashing issues but the damp remained.

A few months later the lead valley on another part of the roof needed replacing so I asked the roofers to look at the chimney flashing and he advised it was all okay. But the damp still existed.

A year later still in improvement so I had somebody in to address any pointing problems on the stack. But the problem still exists a year on.

I'm not sure if we have penetrating damp or just a humidity / condensation issue.

I was in the attic today with a damp meter, cheap Draper one (audible meter) i get steady (damp) pips from the meter all over the woodwork and dividing single skin wall to the neighbours which is obviously not external facing.

On the chimney wall the meter screams a continuous high pitch on the chimney wall, both red brick and the stone. So it's wet/damp all over. Could this be just condensation on the external wall???

Where do I go next? rebuild the stack? silicon water-proofer?

Do you use the chimney?
Is it ever warm?
Does the damp go away in the summer?
Is it only there in the cold of winter?
Does the window in the room have heavy condensation?

You can see from the pictures that the chimney did not have a tray fitted.
If the bricks and mortar are porous then the water would sink down through the wall. The tray would have gone right through the chimney, causing the rain water to run out at a point level with the lead flashing at the front lower edge of the chimney. No tray means the water sinks down inside the wall.

This can be cured by painting the chimney with silicone paint, three or four coats will seal it for more tham 30 years. Once sealed the chimney will shed rain water, it will also breath, allowing the damp to come out.

If you do not use the chimney then it should be sealed.

At the moment, with an open top, you have cold air dropping down the chimney and air warmed by the room, rotating and moving warmth from the room to the outside cold wall.
At the same time, the returning/rotating cold air enables even more warm air to move to the outside wall. And makes the room cold.

It could be that if you turn the heating off or down on a regular basis, the water vapour that is displaced by the temperature drop makes a bee line for the cold surface of the chimney to condense. If the chimney is already damp and cold from the water sinking down through the wall, this will make the problem worse.

In the long term if you do not use the chimney it would be best to remove it.
This may well spoil the classic shape of the home.

Once you have solved the problem ie; it is water sinking down through the wall? Then If you must keep the chimney, you should consider that as a single skin wall, it will always be a heat sink and will cost you a lot of money over future years. The solution, knock the plaster off the chimney breast, cover it with at least one inch thick polystyrene and re plaster.
The chimney will then have a similar value to the rest of the room and any water vapour in the room will land on the window glass.