This a quote from the guy that owned the one at Brooklands - It would seem that the rear wheels should be different so maybe a red herring but possibly worth a look :
I have a very rare prototype I might bring if I am not working that day.
It was one of two built as a design for an offroader C5. Two batteries power two motors. Fat off road wheels at the rear, and joystick steering rather than the bar under your legs. No pedals.
this is a communication from a few years ago with Peter Milner who worked at Sinclair.
Dear Michael,
Good to hear from you. From memory (which is not the best!) we built two
what we called at the time 'off-road' prototype C5s. They had joystick
steering, large section wheels and tyres (rears at least) and the prime
target market intent was the golfing community. The first vehicle was a
purely experimental conversion and the second was closer to the
specification we would have marketed. I don't recall what happened to
the first vehicle but the second finished its days (as far as I know,
because I delivered it) in Connecticut. At least one of the two (and
certainly the second) had a prototype dual-winding single motor offering
'low' and 'high' range.
Some documented information exists, including video footage of my son
driving one of the vehicles on Nuneaton golf course (and through a
perimeter ditch) but it is currently on loan to Roy Woodward (roy@disney
landparis.worldonline.co.uk) who hosts a C5 Internet page. I suggest you
contact him directly if you would like to follow up on this.
Let me know how things progress.
Best regards,
Peter Milner