Right, finally got around to fixing it.
Applied a small amount of Leatherique Prepping Agent and once dry sanded the spot gently. Then applied some Leatherique Custom Colour Dye with a cloth, just dabbing a very small amount of dye onto the leather. I then repeated the dye application each day and 4 days later.... hey-presto, mark is gone. Almost. The matching of the dye colour to my leather (from a headrest I left with the guys at Permanent Painted Coatings) is so close you can hardly see the difference. The trouble is that because I know where the mark was, I instinctively look straight at it when I open the door. All said and done I'm still quite pleased with the result.