Fred99:OTOH the situation described by the OP that someone reversing out of park and hits a passing car, I'm gobsmacked that anybody would consider it reasonable that the driver going past was 50% to blame. Unless there's something much more to the incident than as described.
I've wasted a few minutes trying to find a Disputes Tribunal case that I've quoted in the past but no luck. The search on the DT site is useless. A car reversing out of a supermarket car park was hit by another car that came in through the exit and drove the wrong way around a one way system. The "wrong way" car was at fault.