This was another mystery plant. When I first arrived there were just the dead stalks which I thought were just another patch of the yellow loosestrife. However when it started growing, it was clearly something else.
Now that it has flowered I am fairly sure that it is a Lychnis chalcedonica. It is very different from the other Lychnis species that I am familiar with as it is tall with a thick hairy stalk.
It is inconveniently situated under the Pyracantha so you have the flower heads reaching up through the branches. The tree does provide support which it might otherwise need to stay upright in the strong winds that we have been getting recently.