Cleaning one of London's huge reservoirs while keeping the water flowing

Tap above to watch video report by Martin Stew

They are the filled with the same amount of water that would be used in 8,000 Olympic-sized swimming pools but chances are you may never have spotted the giant reservoirs along the River Thames.

One of them is in Walton-on-Thames and stores around 10 per cent of all water supplied to London every year. Understandably servicing it can get complicated.