On my Plus 8, when the inertia switch is activated, the engine warning light comes on. Initially I expected it to provide a warning when the ECU identified trouble, but there is (or was) no MIL light, the purpose of the light is to provide a warning that the inertia switch has activated. Probably a common practice on Rover engines of the time. I have heard of people having the inertia switch activate when they hit a bit of rough road, maybe the sensitivity of the switches varies.
