A question I may be able to answer!

I got this comment yesterday fron "Alex" on my window repair post from last year.

I realise the trail has gone cold but can you help with a formula for making trowelling mastic to seal the space between a sash window frame and the stonework including under the sill? I recall the main ingredients are (sharp?) sand and linseed oil (boiled?) but don`t know the proportions or if any other ingredient is involved. The shops here only want to sell silicone cartridges and suchlike so your help would be appreciated.

Here is what I can tell you. From the 1909 Household Hints and Discoveries book I have;
Under the index "Putty" I found this.

Hardwood floor filler- The best filler consists of ground quartz mixed with linseed oil about as thick as white lead paint. The particles of quartz are angular and adhere to the grain of the wood.

and this;

After the priming coat is thouroughly dry, putty up all nail holes, dents, cracks and other defects in the surface with pure linseed oil putty composed of equal parts of white lead and whiting. Nearly all the putty sold at present is made of other oils than linseed (chiefly products of petroleum) and ground cliffstone sand. The use of such putty is the explanation of the yellow nail holes and cracks so often marring what is otherwise good work.
The addition of 1 part of powdered litharge to 5 parts each of white lead and whiting in the composition of the white lead putty above specified is permissible, and where convenient, advised.
The addition of the litharge assists in the drying and hardening of the putty.

Your other options would be to mix some lime plaster using sand and masons lime at about 2 parts sand to 1 part lime and using a tuck pointing tool to push into the gap. If the gaps are deep you can plug them with fabric, newspaper or fiberglass insulation. Another option is to use a can of expanding foam insulation to fill the space, cut away the excess and then use joint compound, lime putty or painters caulk to smooth over the surface. If your walls are stone and you don't want a messy junction at the seam, use masking tape on the stone to get a straight crisp line. You may also be able to apply a thin strip of additional molding to hide the gap.

I would be inclined to use the expanding foam and painters caulk myself because it insulates and seals without the threat of cracking later. My next option would be plaster and then painters caulk to smooth and seal the surface. Painters caulk is acrylic based and will smooth nicely with your finger or a damp cloth or sponge. I don't know anyone who inspects the edges of windows to see if the filler is authentic to the age of the house, especially after it has been painted or coated with shellac!