Anna Kettle wrote

What is the "ready mixed" lime mortar you can get from people like Tilcon or
RMC
and how does it differ chemically from lime putty mortar?

all the suppliers' literature I've seen seems to assume it is going
to be mixed with cement on site.

This website shows what I mean:
http://www.mortar.org.uk/wm_hom01.htm

The mix which this company sells is probably hydrated lime and sand.
When the cement is added, there is just a normal cement set, with the
lime acting as a plasticiser for the mix

But had the cement not been added, yes the ready mix is chemically
similar to lime putty mortar - just add water -

Thank you Anna. So if they are chemically similar, why is it necessary to use
lime putty rather than simply mix sand, hydrated lime and water?

Peter