Chimney pots concreted over, ventilation problems?

[jonny0987]

We've recently bought a house and we're having trouble with the external wall that the chimneys on in the top bedroom with damp. There were brown spots on the wallpaper and where the wall joins the ceiling and there is a small semicircle of yellow-brown staining. The chimney breast is behind where the problem is, the wall also has cavity wall insulation.

We got a roofer out and got some repointing done to the chimney and a couple of roof tiles were cracked around that area fixed. I got a dehumidifier and had the rads on for a week painted over the stain where the wall meets the roof and it still came through the paint. Used gloss paint and hasn't come through again.

When the roofer was looking at the chimney he told us that the chimney pots had been filled with concrete. The bottom of the chimney is also boarded up, could the issue be ventilation in the chimney breast? I've never heard of anyone cementing the chimney pots before.

Unfortunately, I don't have an image the ceiling stain as it's now painted over. Would be great to get a professional opinion on this