London is a port city, and Britain became great because of its navy and ability to trade across the globe. The capital’s wealth is built upon that sea trade. There are reminders of London's maritime history everywhere. In Trafalgar Square, we commemorate the 1805 victory (and the death) of Admiral Lord Nelson at the Battle of Trafalgar. At Tower Hill, we remember the 36,000 merchant mariners lost at sea with no known grave. And the Museum of London Docklands examines the triangular trade in goods, sugar and slaves, upon which so much of the city’s wealth was built.

Which is the most important clock in the world? Many visitors to London would answer ‘Big Ben,’ even though this is officially the name of the bell behind it rather than the clock itself. However, as a London blue badge guide, I would say that the world’s most important timepiece is the John Harrison H4 which can be seen in the Greenwich Royal Observatory museum near where the Prime Meridian is marked on the ground.