First of Big Ben's three big birthdays this year arrives

Big Ben is marking the 150th anniversary of the first striking of its massive clock, on 31 May 1859. In fact, it is the first of three such anniversaries this year.

On 11 July it will be 150 years since the Great Bell - its nickname Big Ben eventually became commonly used for the clock tower - first sounded out across London. And September will see the 150th anniversary of the first time the chimes rang in full.

"I think we really do take it for granted sometimes," said Mike McCann, the Parliament maintenance manager who carries the title Keeper of the Great Clock. "But the rest of the world becomes very interested when something happens to Big Ben. It is a unique symbol of Great Britain."

Mr McCann confesses that he too sometimes forgets the symbolism of the building. "It's a funny job. You go about your day to day responsibilities around the Palace and then you get these special events and realize how much it means to people."

He cannot forget about the clock for too long. Once a week he has to climb the 292 steps - they are still talking about putting in a lift - "to make sure it is still there," he jokes.

He has three specialized clock engineers who climb the steps three times a week to wind the clock. They had to use hairdryers one New Year's Eve to thaw the frozen bell hammers in time for midnight. One engineer, Paul Robertson, walked from his home during the February blizzard as he knew Big Ben would stop if he did not wind it.

A further 30 or so steps further up is the Great Bell and the four smaller bells which sound the quarter hour chimes, all of which are triggered by the clock on a mechanism. This week, Mr McCann has been supervising the cleaning and restoration of the bells in time for the anniversary.

The smaller bells collectively play a tune, the Westminster Chimes, based on Handel's Messiah, the full version of which is heard on the hour. This is followed by the famous "Bongs" - the first of which signifies the hour.

The 300ft-high clock tower was constructed as part of the rebuilding of the Palace of Westminster after it was destroyed by fire in 1834. Charles Barry, the architect subcontracted the design of the tower and the clock to Augustus Pugin.

The tower has withstood the elements, survived the Blitz and emerged gleaming from the restoration of the Eighties. Although Mr McCann admits the clock does stop occasionally, its most famous interruption occurred in 1944 when a flock of starlings perched on the hour hand.