Thanks so much for the ID and advice. I read up on this plant and definitely will not plant it. I know from sad experience what an invasive plant can do to a garden! Marian

That makes sense - I was at a distance shooting the blue-gray bird with a smaller lens and by the time I reached him, he flew into a thicket and when I looked into the thicket I found the catbird. They didn't seem like one and the same bird. Thanks for the IDs, TMFF, and clearing up my confusion. Shyamala

I would go with one of the many varieties of graptosedum, or sedeveria.

Well, Google is not indicating it as necessarily rare. And I did assume it was growing wild, however river banks do occur on private property and adjacent to private gardens...... But I will always defer to a native for identification of any UK plants.