In the shadow of the cleaner...

It is the first time the clock has been cleaned since 1995.

The cleaners have a big job - the four dials of the clock are 23 feet (6.9 metres) square and have 312 panes on each face; the minute hand is 14 feet (4.2 meters) long and the numbers are about two feet (500cm) high.

At that height the cleaners may have to move between the different faces to keep out of any strong gales.

But tourists need not be alarmed - the cleaning will not disrupt the clockworks nor Big Ben's hourly gong.

New Year delayed

The "Great Clock" first began recording time in 1859.

It was named after the 13,760kg (30,272lb) bell that marks the passing of each hour - named in turn after Sir Benjamin Hall, British Commissioner of Works at the time of the clock's construction.

Hazards for the cleaners include high winds

The tower that holds the clock and bell is actually called St Stephen's Tower.

It was designed to be accurate within 1.5 seconds, and experts say it has very rarely strayed.

It withstood about a dozen attacks by German bombers during the Second World War.

But more mundane attackers have slowed or stopped the clock on occasion.

In 1949, a flock of starlings perched on the minute hand, slowing it by 4.5 minutes.

Snow caused the clock to ring in the new year 10 minutes late in 1962.

And mechanical problems have stopped it three times - once in 1976 and twice in 1997.

Since 1923, Big Ben's chimes have been broadcast by the BBC with its headline bulletins.

If an old English penny is added to the pendulum it gains two fifths of a second a day. We have yet to work out what a euro will do

Thwaites and Reed, Big Ben maintenance

It was a staple of TV viewing when it opened the now defunct ITN News At Ten.

Thwaites and Reed, the company which maintains the clock, has staff on 24-hour call should something go wrong.

Workers visit the tower to wind up the clock three times a week.

Timekeeping is maintained with weights, which include an old penny piece, moved along the pendulum to alter the speed fractionally.

Thwaites and Reed say adding an English penny to the pendulum makes it gain two fifths of a second a day - but they have yet to work out what a euro will do.

In September last year hundreds of tonnes of grout were pumped underneath the tower to stop it from wobbling during excavations for the Jubilee Line tube extension.

The tower moved by between 17 and 25mm during the construction work, said experts, but was never at risk of falling over.

The clock tower wobbles naturally by nine millimetres a year as a result of seasonal factors, and has always tilted slightly to the north west, a slant that is just visible to tourists.