After much thought and many conversations with local bricklayers we've decided to rebuild half of the original chimney. It will still need to be over 4m high but by only rebuilding the section with the four flues we'll only need about 60% of the bricks and mortar. We'll also be able to get rid the gully tunnel that runs under the chimney and replace it with a more sensible gully between the two pitched roofs and step flashing all around the new stack.
Since we still need a chimney for one of the flues that isn't being rebuilt at the back of the house we'll install a twin wall flue and fix it to the rebuilt stack. I've attached a photo of a sketch that we came up with in conversations with a local stove fitter. This seems like a best solution to a tricky rebuild. Eliminating the gulley that runs under the chimney stack seems preferable to reinstating it which always seemed to me like a less than ideal solution.
We're going to supply the NHL 3.5 and sand. The bricklayer has said that he's going to use a mix of soft and sharp sand. I think he said he'll use a 2:1:2 mix of soft sand:sharp sand:NHL 3.5.
What does the PPUK hive mind think about that mix? I thought that it was best to use sharp sand only but having never built even a garden wall (yet) I obviously am reluctant to dictate to him what mix of mortar he should use. I would welcome your thoughts on what mix you think would be best for a chimney stack in an exposed location very close to the sea.
The bricklayer has also suggesting parging the flues. He suggested doing it in sand and cement which strikes me as odd. What do think? Is parging necessary for a chimney stack that will be lined?
Also, is there a consensus on what height chimney pot we should use for a stack of this height? We'll need to buy four since the ones that were used previously have seen better days and are probably best retired to the garden. We have an angle on four matching reclaimed ones that are 550mm tall but I'm wondering if perhaps they won't look a little squat on a 4m high stack. It is entirely possible that I am overthinking this, however.