Damp ingress on chimney wall?

Hi All, Newish home I'm currently getting a damp patch on my upstairs chimney wall. The facts are:

-Had damp issues from day one
-Unused chimney open in bottom reception room to allow ventilation
-Checked flashing and all looks good plus the chimney stack in the attic is dry and shows no signs of damp.
-Put a Cowl on top to stop rain ingress but still allows ventilation. (No effect)
- Was signs of damage to render at the bottom of the stack above the roof which I had hacked off and re-rendered with a belkast drip bead. Once set and dried out I Clearcoated the render. (I though this had done it but it may have just dried out abit with the hot weather we had for one week in the summer!)
-Pointing on chimney was carried out before I bought the house and is in very good condition
-No signs of water ingress anywhere, attic inc except for when it shows up on my recently plastered wall? (Pics attached)

Have run out of ideas now and a tad peeved! Any suggestions or guidance would be greatly appreciated.

Cheers
G

Google 'hygroscopic chimney salts'. If you have run out of ideas and everything is sound on the outside it is more than likely that.