Mystery Mini derivative is a Napier. But what's that?

First there was this sketchy group picture with a bright green Mystery Mini Derivative (click here) in which reader Colin Baines recognized himself. Then there was a better picture of the thing dug up by Kees Plugboer which gave the registration away and with that a bit more info, too (click here). Now it's time for another shot, from Kees again, showing the rear of the car. 

The DVLA previously learned us that 'Q412 NTR' used to be registered as a 'Tourer' of 'Other British Makes' in July 1984, but by this time the system has it as a 1300cc Napier! Does that ring any bells to anyone here? I also noticed that it is remarkably close registered to another Mini derivative: a Langridge Navajo with the number 'Q413 NTR' (more here) and in an equally bright green colour, too. A coincidence? Or do these two cars have something in common after all?

First picture of the rear end of 'Q412 NTR', which turns out to be a Napier 1300

Picture courtesy Kees Plugboer

The only pictures of the thing date back to 1984 - also the year it was registered 

Picture courtesy Kees Plugboer

This Langridge Navajo must have been registered on the same day. A coincidence?

Picture Jeroen Booij archive