I note that you say in your damp and damp proofing section "Outside flues can sometimes cause condensation on the external wall they rise on, check the masonry in this area. (Make sure the boiler is off for this check)."
I had my boiler serviced last year but have recently noticed a white line running down the wall from the flue where condensastion must be forming and there is a damp stain on the bottom brick which is now starting to dry out as the weather is warmer.
I can't tell if it has affected the internal wall as it is behind a sealed kitchen unit.
As far as I can see none of my immediate neighbours have the same problem so what can I do about it?