As keeper of the Great Clock, Mike McCann regularly walks past a wooden plaque on the wall alongside the mechanism which regulates the sonorous boom of Big Ben. It reads: "All through this hour, Lord, be my guide/ That by Thy Power/ No foot shall slide."
The line, which is from Handel's Messiah and forms the musical basis for the chime that emanates across London from the Palace of Westminster clock tower every 15 minutes, will have particular resonance today for Mr McCann and a team of anxious engineers.
At 8am precisely, the four "quarter bells" responsible for producing the familiar "ding, dong, ding, dong, ding, dong, ding, dong" which reverberates around Whitehall four times an hour will fall silent for the first major repair to their Victorian workings since 1955.
A specialist team will spend about four weeks - considerably more than the hour cited by Handel - carrying out a delicate repair to the largest of the four quarter bells, hoping that no foot, hammer, wrench or other tool will slide or damage the mechanism that has tolled the bells more or less perfectly since their installation in 1859.
Big Ben, which is actually the name of the 13.8-ton main bell that provides the Great Clock in St Stephen's tower, the clock tower with its famous hourly "bongs", will continue to function as normal. But any procedure to tweak a clock, which is a symbol of parliamentary democracy and a capital city - if not a nation - as much as an outsized timepiece and marvel of Victorian engineering, carries with it a heavy burden of responsibility.
Mr McCann, who also goes by the slightly more prosaic title of maintenance manager of the Houses of Parliament, said: "On most days, the clock is something that is just part of the job. You make sure it is running accurately and ensure the normal maintenance is correct.
"But when you do something like this, it is more stressful. It's always nerve-racking when you stop a part of the mechanism and dismantle it. It's even more stressful when you restart it and hope it all works."
The interruption of the Westminster Chimes, as the tune based on the Messiah is called, is due to an alarmingly worn bearing in the yoke, or arm, that holds one of the two hammers used to strike the fourth and largest of the quarter bells, weighing some 3.6 tons. The damage to the bearing was noticed in October last year during routine maintenance to the bells and has been carefully monitored for the past eight months.
Mr McCann, who assumed the biblical-sounding title of Keeper of the Great Clock earlier this year after the previous incumbent retired after 25 years, said: "We had to work out the rate of wear to this bearing. If it had been there since 1848 and become that worn then we could have waited 10 years to carry out this repair.
"But it is actually getting much worse very quickly so we need to get in quickly."
He also confirmed that, although the work is scheduled to last for four weeks, pedestrians in Whitehall and listeners of the BBC, which transmits the chimes live from a microphone in the belfry linked to Broadcast House, may have to wait until mid-July before the chimes are restored.
Mr McCann said: "There is no text book with 'How to repair Big Ben' on the cover so you can turn to page six and see how to replace this bearing. There is no one still alive who carried out the 1955 repair. You literally have to make it up as you go along with something like this, using people who know what they're doing. Four weeks is our best guess but it could be as little as a fortnight or as much as six weeks. We simply won't know how long it will take until we have stopped the mechanism and had a look."
The absence of an instruction manual for the contents of the 96-metre-high, neo-gothic clock tower, designed as the crowning glory of the new Houses of Parliament by Charles Barry, after the old palace was destroyed by fire in 1834, is testimony to the ad hoc nature - and dispute-ravaged progress - of the project to build it.
When Barry's design for the new parliament was unveiled - and after he had infuriated the horology trade by inviting just one clockmaker to make the iconic time piece - a demanding specification was drawn up by the Astronomer Royal, George Airy. Mr Airy wrote that "the first stroke of the hour bell should register the time, correct to within one second per day, and furthermore that it should telegraph its performance twice a day to Greenwich Observatory, where a record should be kept".
Most master clockmakers of the day complained that such a level of accuracy was impossible for a clock of its size. The best that could be hoped for, they said, was to be correct within three minutes per day. Even by the standards of modern engineering, the demands of Mr Airy were steep and the Victorians were pushed to the boundaries of time-keeping technology to achieve it. The clock that was eventually installed in a wind-proofed steel chamber below the five bells is a 5-ton clockwork behemoth.
It works on similar lines to a grandfather clock, powered by the force of gravity and a pendulum with three stone weights - each weighing the equivalent of a Mini - hanging from it at about 300ft from the ground.
Crucially, it is accurate to within one second per day and as such Big Ben remains the largest and most accurate striking mechanical clock in the world.
Such finesse was achieved as the result of a remarkable and fiery collaboration between Edward Dent, one of the foremost clockmakers of his day, and Edmund Denison, a leading barrister and renowned gentleman amateur of the science of horology. It is Denison, who later became Lord Grimthorpe, who is credited with inventing the mechanism that makes Big Ben so reliable - the double three-legged gravity escapement, which, in broad terms, isolated the pendulum of the clock from the forces created by the 400kg hands of the clock, designed by Augustus Pugin.
Denison was put in overall charge of bringing the clock to fruition and his blunt manner led to frequent clashes with others involved in the new parliament building. Dent had finished making the clock mechanism in 1854, some four years before the Great Clock Tower was complete, allowing Denison to tinker with his design in the hapless horologist's workshops. The peer also became involved in the design of the bells for the clock, in particular Big Ben. Until the Westminster clock tower, the largest bell ever cast in Britain was Great Peter in York Minster, weighing 10.3 tons.
But Denison was adamant that his own design, method and alloy recipe would allow a larger bell to be created. Eventually, a 16-ton monster was cast at the Warner & Sons foundry in Stockton-on-Tees in August 1856. While it was on display in the New Palace Yard at Westminster it cracked beyond repair because the 300kg hammer specified by Denison was twice the specified weight.
The main bell was then recast at the Whitechapel Bell Foundry, where this month's repair will be carried out, with lower weight of 13.8 tons. It took minutes to fill the mould with the bronze alloy and 20 days for the metal to cool and solidify.
It was this "Big Ben" which was installed in the Westminster belfry and began ringing on 7 September 1859. It is this bell that continues to ring today but within a month of its installation it was also cracked by the same 300kg hammer demanded by Denison - and criticised by George Mears, the owner of the Whitechapel foundry.
The dispute went public and resulted in two libel cases against Denison, who was found to have befriended one of the technicians at the foundry, got him drunk and bullied him into giving false testimony that the fault had been due to poor workmanship and concealed filler. The cantankerous lawyer lost both cases and a close examination of Big Ben in 2002 found that there was no filler in the bell. As one contemporary of Denison put it: "Zealous but unpopular, self-accredited expert on clocks, locks, bells, buildings as well as many branches of law, Denison was one of those people who are almost impossible as colleagues, being perfectly convinced that they know more than anybody about everything - as unhappily they do."
Whether Denison's expertise was responsible for designing the faulty bearing at the centre of this month's urgent maintenance is unknown. Certainly his creation has stood the test of time. Other than for essential maintenance, the clock and its bells have only fallen silent five times in the five million or so times they have rung since 1859.
Mr McCann said: "It's a remarkable piece of engineering really. There can't be many mechanical objects out there dating from the middle of the 19th century still working as well this clock. Admittedly though with a lot of tender loving care."
Westminster weights and measures
* The clock tower at the Palace of Westminster is often called Big Ben but the name actually refers to the main 13.8-ton bell
* It takes 54 seconds for the bell to sound the 12 'bongs' at midday and midnight
* The chiming and striking mechanisms have been powered by an electric motor since 1912. Before then, it took two men 10 hours to wind all the mechanisms
* The main bell was damaged in 1859 by an overly heavy hammer. A square piece of metal was ground out of the cracked area and the bell rotated through 22 degrees. The crack is credited with giving the bell its distinctive tone
* The combined weight of the five bells is 21.4 tons. They sound the notes E, G sharp, F sharp and B
* The longest silence of the bells was in 1976 when the chiming mechanism suffered a breakdown. Repairs took 10 months
* The clock is adjusted by adding to or subtracting from a pile of pre-decimalisation pennies on the pendulum. Each coin adds or subtracts a second
* The clock tower survived the Second World War unscathed but a flock of starlings managed to stop the hour hand in 1944 by perching on it