I asked this question on the NG some time ago but the suggested
solution of rebuilding in the same fashion does not appeal.

Would it be feasible to use "casing" or a couple of wood sheets nailed
approx 750mm apart, then fill thes with a gravel/ cement mix along
with most of the original stone.

My thoughts are that the more stone that goes in the stronger the
final wall.

Or might there be problems with the ingredients expanding and
contracting at different rates?

Regards

HN