Logic says the spire of Salisbury Cathedral should have fallen down before it was even completed but it’s still there and, 700 years later, Kevin McCloud is going to climb - through 14th century scaffolding and over crumbling, eroded stone - to the very tip. A waterlogged site and just four feet of foundations don’t seem the ideal conditions for building a 404-foot spire - the tallest ever constructed at the time. Today, the enormous structure still stands on gravel and water; shored up by metalwork added as early as the 14th century. Kevin’s climb - the highest in the series - is made more difficult by crumbling stonework, eroded by acid rain.