Best DIY way to tackle this...

digannio

stuart45 said:

Bricklayers sometimes used to close the cavity at the cill by cutting 4 inch blocks to lay flat across the cavity. Without a DPC, this occasionally allowed moisture to track across to the inside.

The problem area is the plaster just below the cill and nowhere else on the wall and is limited to two spots of salts (you can see a crystal type reflection with a torch). When there was a tide mark it was also just below the cill as if water was getting in through the sealant which isn't the case. It must have been getting absorbed through from the external wall because it's stopped since I treated it with the cream.

stuart45

It could well be tracking through the stonework just under the cill, as that course looks wet in the photo. Usually the overhang from the cill protects below from the weather. If the damp treatment is working see how.It goes.

digannio

Thanks for your help. The darker colour on the stonework is from the application of the waterproof cream which darkened it a bit but is slowly returning to lighter colour.