The New River is neither new, nor a river. Built when Shakespeare was alive, the aqueduct is Thames Water’s oldest man-made asset, pre-dating some of London’s more famous historic sites like Buckingham Palace and St Paul’s Cathedral by decades.

More than 400 years later it remains a vital artery, supplying water to 700,000 Londoners.

The 25-mile river rises at Chadwell Spring in Hertford and ends at the East Reservoir in Woodberry Wetlands, Stoke Newington, which was reopened to the public in 2016 by David Attenborough.

Charged with maintaining it are a dozen or so Thames Water workers, one of who, Gary Stephens, grew up by the river in Enfield and even swam in it during long hot summers.

Aged 18 Gary got a job on the river with the former Metropolitan Water Board.

"To really get to know this river you have to let it get under your skin, you have to walk it, you have to feel it and you have to live it," he said. "When I was a kid I used to swim in it. When I was 18 I got a job here and I said to myself: 'I'll give it six months.'

"That was 42 years ago.

"We are the custodians, the caretakers, of this river. We have a duty of care towards it and we strive to keep up that important stewardship to the best of our ability during our time."

The river is an incredible feat of early 17th century engineering. Before it, London's water supply was limited to the Thames, streams, wells and springs, which were often contaminated. Water was distributed by people carrying buckets.

In 1606 a Parliamentary Act gave the Corporation of London power to make a "new river" from Chadwell Spring to north London and, in 1609, the job was given to Hugh Myddelton, a goldsmith and merchant adventurer who built it over four years.

More than 200 labourers were paid the equivalent of 4p a day to dig it out, and skilled carpenters earned 6.5p a day to wharf the banks and build bridges.

The water was then brought to the city's streets through 400 miles of hollowed-out elm pipes from ponds at New River Head in Islington. The cost of the construction was £18,500 - about £5million today.

Its route follows the Lee Valley, falling only a few centimetres as it meanders through Broxbourne, Cheshunt, Waltham Abbey, Enfield, Southgate and Hornsey. It used to end at New River Head in Islington, but the route has been shortened over time.

The original spring provided 10 million litres a day to the capital but, as demand grew during the Industrial Revolution, it increased to 102 million litres a day when it was connected to the River Lea in the 1700s. That doubled in the 1800s with the construction of pumping stations to abstract water from deep wells.

A series of boreholes were dug in the 1990s to enable surplus treated water to be stored and then pumped into the river when extra water is required.

Today it provides 8 per cent - 220 million litres a day - of London's water, and is enjoyed by walkers and wildlife lovers.

Frankie Somers has been at the helm of the team for a couple of months.

"One of the first things that struck me is that this is not just to supply fresh water to London, it's a living, breathing, vein through the city," he said.

"I was blown away by the biological diversity. I have seen kingfishers, swans, geese, dragonflies, water voles, newts, bats, woodpeckers and parakeets, and magnificent architecture.

"I cannot emphasise how important it is for the people who grow up around it. 65 per cent of it is open to the public. Inner city schools are encouraged to come along and learn about wildlife. It's as close to the countryside as some of the children have ever been.

"I have worked in the industry for 17 years and I had never heard of the New River.

"It's like our best kept secret."

Frankie and the team spend the summer clearing weeds - including one particularly virulent species that can grow up to a foot each day - that can cause blockages and flooding.

Twice a week they walk the length of the river checking for leaks and erosion, clearing any other debris from the water, and checking the level of the boreholes.

"It's the ultimate challenge," Frankie said. "Getting water from A to B in the safest way possible. Too much water could cause flooding, not enough could lead to supply issues. It's like spinning plates."