The one I remember looked like an overgrown Volvo [Simon Templar] sports car. It had a monstrously long bonnet with a RR radiator. It was definitely a RR Merlin aero engine in it [I saw it] and there was certainly no mistaking the sound; he could only run it for a few minutes stationery before it began to overheat.
RR did, indeed, threaten legal action to get him to remove the grill - until someone pointed out to them the publicity value. They promptly became sponsors and supplied much technical help.
It used to be taken for regular burn-ups on the Autobahns, where the Porsche-driving German Police could not catch it. However, the story goes that they created a roadblock to stop it - just to admire it!
It burned out during a charity visit to Norway...