I can't say for certain, but recently I needed to set new glass into a
stone mullioned window. I made up a 3:1 mix of silver sand (labeled as
block paving sand at Wickes) and lime putty. Made it a lot easier to
get a good finish, has bonded well to the limestone masonry and so far
has proved stable.

On one occasion I also made up a mortar using building sand rather
than sharp sand (all I had to hand), and used it point in a wooden
window frame with quite satisfactory results. The usual sharp sand
lime mortar mix can be tricky with closely-jointed masonry, so I
wouldn't be afraid of trying alternatives for difficult locations.
Unlike portland cement, lime mortar can easily be knocked off if
there's a need to change it.