The Great Westminster Clock is the largest striking, most powerful and most accurate public clock in the world.

The clock tower is 315ft (96 metres) tall and made of Anston stone from Yorkshire.

There are 292 steps to the clock room and a further 101 to the lantern.

The clock is a unique flatbed design weighing about five tonnes with a pendulum which beats every two seconds.

Pre-Decimal pennies are used to adjust the clock timings. One penny will add around two-fifths of a second in 24 hours.

The clock faces are 23ft (seven metres) in diameter and framed in cast iron.

Workers are rumoured to have slept on these ledges as they kept the original gas lamps burning through the night.

Twenty-eight, 55 watt, energy saving light bulbs illuminate the four faces.

Big Ben is the name of the hour bell. It measures 8ft (2.7 metres) in diameter, is 7ft (2.2 metres) high and weighs 13.5 tonnes.

A microphone near Big Ben picks up the time used by the BBC for worldwide broadcasts.