Well, I posted my query also on a Norwegian site and received an answer from a botanist today: The plant on the right is not a form of Mertensia, but Honckenya peploides (syn. Arenaria peptoides), Sea Sandwort or Seaside Sandplant. Really quite excited by this as I'd had this one of my wants list for some time. Over much of its range it was formerly used locally for food, most often the leaves were fermented - I have records of this from Scotland, Iceland and the Aleutian Islands....
I took the plants in the picture home (Mertensia and Honckenya) and planted in sand mixed with seaweed compost and the plants have thrived.