Bath stone mortar mix

If nothing is directly forthcoming, you could try contacting the Cathedrals Workshops Fellowship. They'll be able to put you in touch with somewhere that uses Bath stone and a quick word with the masons will get you the info.

You should only need it for the visible surface, not for the bulk of the mortar (although you haven't said what you're doing with it).

I'm building an extension. I was going to build the inner leaf of the cavity with standard blocks and mortar, and then the outer leaf with reconstructed stone blocks and Bath-stone mix. Do you mean that I should build the external walls with normal cement as well, and then just point the gaps afterwards with the Bath stone mortar? How much gap would I leave for pointing if I did that?

thanks,

dan.

I don't know what's recommended for such stonework

That's a possibility.

Obviously this adds labour costs (or time overhead if you're doing it yourself), and you need to contrast this with the savings on mortar. If the sones are large, then they won't use as much mortar as, say, bricks would.

About the same as the width of the mortar.

Another technique which is used is to blow brick dust of the right colour (maybe mixed with cement, not sure) onto the wet pointing. I guess this might not last so well. I saw a brickie doing something like this when moving a boiler flue. He drilled the new hole and saved some of the brick dust. The old hole was filled with a dollop of mortar and smoothed off. He then blew the saved brick dust onto it, and finally recut what would have been the mortar pointing through it. I can't see where this was done now -- it's completely invisible on the wall.