Some of the best advice I've ever been given about visiting London is to look up. The architecture and artistry will always surprise you. A great example of this is the Water Clock in Neal's Yard. This marvelous example of aquatic horology sits high up on the side of the building where I now work. I didn't even notice it for the first two weeks.
After some investigation I found the clock was commissioned in 1981 and created by Andy Plant and Tim Hunkin. As well as being an engineer, Tim is a cartoonist and created this wonderful poster to go along with the clock. Sadly this unusual timekeeper is yet another of London's Stopped Clocks but I did find this great piece of video of the clock in action: