This is why I also leave the car in gear when it's parked - normally 1st. If the car's pointing downhill even slightly, I leave it in reverse.
Also when I park on hills, I turn the wheel into the kerb. That way you've got the handbrake, gearbox and kerb stopping your car from going anywhere.
People still do this? I always leave my car in gear when parked, 1st or reverse will offer the best anti-rolling resistance. I also always start a car with the clutch depressed (negating the need to check if it's in gear) as it slightly reduces the starting load.
Does the manual or highway code not explain how to park safely on a hill? I thought these were 'common sense' though I'm finding common sense is increasingly uncommon.