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.
UPDATE 27 May 2021: Found! Click here