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My mother resides in a traditional sandstone property in a coastal area of West Central Scotland. The West gable has serious water ingress problems, and requires re-pointing...as do other aspects of the building, but to a lesser extent. The owners have obtained two estimates, and are keen to go with the cheapest. The cheapest tradesman has indicated that he intends using cement with SBR bonding to point the building. I maintain that this is incorrect, and could in fact be damaging to the building. Can you please advise the correct mortar mix to be used?
Many thanks...

Hi
You should always try and repoint with the same mortar as was used when the house was built.

Cement: sand mortar are usually too strong and inflexible for use in older properties. When movement takes place, the mortar does not flex and the bricks or stone blocks crack instead. Also, cement mortars are very impermeable to water, which concentrates water absorption into the bricks/blocks. This saturation of the bricks/blocks can lead to frost damage in severe winters

If the sandstone is very soft, a mortar based on hydraulic lime and sand would be appropriate. Otherwise try a cement: lime (hydrated lime): sand mortar