Checking over my trees yesterday I noticed that the large "wrap-around" root on the big hawthorn appeared to have been killed by the recent carving I had carried out. Something made me give the embedded stone a little push and where it was once stuck fast, the dying off of the root had released the grip. The stone popped out without leaving much of a mark on the trunk and so I popped the other two small stones out too. I gave the big, dead root a jiggle and found that it was not grafted onto the trunk as I had previously thought. With a bang from a hammer, the root was removed.
There is a bit of a mark around the back of the trunk, but on the whole I am quite happy with the result and I never expected the removal of the root and rock to be possible.