On 30 December 1952, a double-decker bus drove on to Tower Bridge on its usual route between Shoreditch and Dulwich.

It was late in the evening, dark and the temperature had dipped below freezing.

It was a couple of weeks after the great smog had brought London to a standstill, and although that particularly foul miasma had dispersed, smog still regularly reduced visibility.

The traffic lights were green, there was no ringing of a warning hand-bell.

Albert Gunter, the driver, travelling at a steady 12mph (19km/h), proceeded on to the bridge.

Then he noticed the road in front of him seemed to be falling away.

He, his bus, its 20 passengers and one conductor were on the edge of the southern bascule – a movable section of road – which was continuing to rise.

It was too late to go back, too late to stop.

So the former wartime tank-driver dropped down two gears, and slammed his foot on the accelerator.