Thx for the recipes, mo.
When doing stone work, even with mortar the stones should always be set with the idea that if the mortar wears away the stones will stay in place. Again, flat stones will help.
Portland in the mix does give strength, and I think that has helped some of the more challenging stone repairs I've done to stay in place. In particular there are doorways in the barns in my area that were not built with care originally, and the sides of these doorways tend to fall in, the rebuild can take some patience to find the right stone or cut the right stone. Add to this that cattle will soon be using the doorway as part of thier fighting arena and one had better wish good luck to the repair.

I wish I could see the buildings you speak of Ken, a visit to the UK remains high on my list of dream vacations...