I've drawn a blank as I, too, have never seen this one before and don't know anything about it. Is it registed 'BNP 858B'? That would make it a 1964 car, which could fit in with the flat roof of the early vans and estates. Or is it 'BNP 858S', which is mated to a Leyland Cars vehicle of 998cc capacity and 1977 vintage..? Anyone out here who knows more about it?
UPDATE 13 May 2021: Jens Lillelund writes: "The rear lights are from a 1967 BMW, so it must have been built from '67 on." Clever! What's more: both Eddie Fitzgerald as Graeme Farr recognized the car, which was supposedly featured in a magazine article in the 1970s. Graeme: "I have seen it before. Either in Hot Car or in Terry Grimwood's column 'TG's Home Brew' in Cars & Car Conversions, I would say around 1970. I'm sure it had a cool custom interior too with either Corbeau eyelets or Rover 2000 leather seats." I managed to ask Terry Grimwood himself, who replied swiftly: "Sorry Jeroen, I have no recollection of that particular car. Rather nice though." And so the search goes on. Graeme and Eddie will see if they can find back the article.