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Sure enough, not so simple on this side of the pond. I see the single fastener but it's not slotted like most quarter turn fasteners. Not sure if it just pulls out with some force, but turning it with my fingernails does nothing. Maybe some one knows?
Often these fasteners, over the years, have been replaced by non OE "anything will do" studs. If it's not slotted then it's not original and is likely to be a Fir Tree type fastener that you might just need to use some effort with a trim remover tool underneath it.