I was standing outside the new Tube station at Shepherd's Bush last week, and marvelling at the regeneration that can be achieved by sensible investment in transport. The proprietor of a local coffee-cum-ice-cream bar was telling me how much better things were going, and how many new customers he was getting from the nearby Westfield shopping centre, when another man seized me by the elbow.

"Mr Boris," he said, in tones of despair and an accent that suggested he was not native to London, "what are you going to do about all this?" As I followed the sweep of his arm, I saw the gleaming London Underground signs, and the capacious concourses, and the happy crowds of shoppers and commuters milling in ergonomically efficient patterns over the spanking piazzas; and then – beneath our very feet – I saw what he was driving at.

Imagine if you were Leonardo da Vinci and you had just immortalised the complexion of the Mona Lisa, and then you came back to find some vandal had used a magic marker to blob black stubble on her lovely cheeks and chin. Or put yourself in the shoes of the head groundsman at Wimbledon, and imagine that you had spent a whole year mowing and watering to produce the perfect lawn and then – on the eve of the men's final – 200 moles had simultaneously erupted through the grass of Centre Court.