I built my conservatory last year, the dwarf wall was made up of brick externally and lightweight block internally which is plastered. From then until now cracks have appeared and got worse in the 2 internal corners furthest away from the house. Also my skirting scribes have also opened up (gutted, I got them perfect as well). One corner has got really bad and the crack and scribe has opened up by about 10mm. The external bricks have absolutely no cracks at all and all mortar joints are intact.
In that case the most likely cause is that the blocks were wet when laid and a strong mortar mix was used. Shrinkage is now occuring and is concentrated in one area because of the strength of the wall.
Sorry - probably not the answer you wanted.
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