I inspected my seats today and have determined I have two holes so I am back to square one!
It turns out not as far back as I feared. I have been in touch with Melyvn Rutter and he has told me that I have the Restall seats that were standard in the '70s. I need to remove the seat and trim and check the hole spacing (by feel it is 6 inches thought the leather and in to the frame).
It seems new headrests and fittings can then be supplied.
