It depends on the original mix. You won't find NHL in the sheds. Most builders merchants stock NHL 3.5 now, and lime specialists stock other types as well.
Looks like you may have a solid wall.
Use a 3/1 sand/lime mix for NHL. If you look at the sands here https://www.limebase.co.uk/sands-and-aggregates you will see that there are various grain sizes. Larger grain sizes are better for walls with really large joints, such as random rubble stonework. Finer sands are better for walls with smaller joints.
Sands for limework tend to be sharper angular grains, whereas for cement mortar a softer sand with rounded grains is better.
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