There are damp patches near the skirting & sides of chimney breast. do

Chris Plastow

This is hygroscopic damp caused by ammonium sulphate contamination. You will most probably have to re render with a waterproof render to 300mm past the visable damp. Test the vent , which should be at floor level, with a lit candle to observe the air movement. It is critical that there is an air flow.