Thanks Woody that does sound hopeful but I can't think why that would be right (I'd be quite glad if it is though). Is that what you have found in practice?
My house is a victorian terraced house (1890 ish)of very standard design and construction, which I was having re-rendered. As the render came off up the back wall and then up the top of the chimney stack, my builder pointed out that the chimney stack wasn't looking too great as the mortar was...