how to repair corner stones which have moved

If this is natural stone, as in stone quoins, then if they are badly damaged cut them out and replace. If the damage isnt to bad, you need a product called Lytho-Mix, from a lime supplier, such as Trad-Lyme, plus colour pigment. Unless you have been doing this type of work before, its going to be trial and error, basically mix the tradlime with water plus add you pigment to match stone colour, and apply to area after wetted down, needs dampening often and troweling. There are other techniques, but if I tell you them, you will be doing us all out of work.

+1 as above, excellent