Well, it turned out to be a beacon, maintained by Trinity House (who were also replacing buoys and depth markers around the island, that day...).
And this is one of the two guys tasked with painting it sparkling white, to keep it so highly visible: who were very friendly,
and happy to chat about what they were doing. It has to be said that they'd done a great job, as well!

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If you're not viewing this in the 'Lindisfarne' gallery,
more piccies of this interesting object can be found here...!