This article by Gerard Lynch was published in 2007.
It’s been ten years and still the ready mixed lime mortar producers haven’t fully taken it on board.
The most enlightening statement for me is this:
“under analysis, the majority of historic lime mortars are not commonly found to be 1:3 but typically vary between 1:1½ and 1:2, just as the original mortar makers and craftsmen intended. This is borne out by extensive analysis carried out over many years by The Scottish Lime Centre Trust. (At the last count the organisation has analysed around 4,500 historic mortar samples, approximately 80 per cent of which were from Scotland, 10 per cent from England with the remaining 10 per cent from various other countries.) The average lime:sand ratio on the organisation’s entire database of historic mortar samples is around 1:1½.”