woodsmith
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Definitely not expanding foam.
I'm not sure how you have a 20cm cavity, is the wall a cavity wall and you are measuring through the cavity into a gap in the mortar on the internal skin?
Generally the mortar is raked out to about 25mm and then replaced with a mortar mix of six parts builders sand to one part lime and one part Portland cement. Any larger voids in the mortar would be filled with the same mix making sure you don't bridge the cavity in a cavity wall. Alternatively you can buy small bags of ready mixed mortar from B&Q etc if you don't anticipate using a lot. One thing worth trying though is to point a small area, leave it a couple of days and check that the colour is similar to the existing pointing, if it isn't it would be worth trying a sand from another supplier.
That is unless the house has been built with lime mortar (you can easily tell if you have lime mortar, it is light in colour and full of large bits of stony sand and often small lumps of lime which stand out white). If this is the case you need to use 3 parts sharp sand to one part lime, but just to complicate things this is not the same sort of lime you use in a cement mortar mix. Lime for lime mortar is called NHL lime but if you tell them at the builders merchant they should point you to the right product.
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