The super chargers did have an 'M' gear and an 'S' gear, which in ' S' gear ran faster to compensate for the drop in air density with altitude. So at low altitudes 'M' gear was selected. Should electrics, hydraulics, pneumatics fail then most items returned to a fail safe position, ready for landing. To clarify 'M' gear was always selected for landing.
Bingo, I reckon that answers the question. Many thanks Cliff, you're a font of knowledge as usual.
Adam