When I started watching motor racing in the 1950s one of the support races at each meeting was usually for 500cc Formula 3 cars. These were mostly Cooper or Kieft machines, but there was always a sprinkling of cars from other low production manufacturers, and one-off specials. The SeeRed meeting at Donington Park in May 2011 included a race for these 500cc cars with a sizeable entry list, amongst which was the car shown here.
It's the 1949 Trimax Monoposto, built by Spike Rhiandro and driven in this race by Duncan Rabagliati, and you can read the history of this car here. The car was designed to allow for engines of 500, 750, and 1,000cc to be fitted (hence the 'Trimax' name) and for the Donington Park race it used the JAP 497cc engine shown below.
Appropriately, it mirrored its debut performance in 1950 at Goodwood by coming to a standstill on the run-off area just past McLeans Corner on the first lap of the Donington Park race.