Photography is often about moments which make unique photos that can't be replicated, not with the same person in them, not with the same light, or the same mood. A famous building or monument is not a moment in itself - it's merely a context anyone with a camera can capture. The following are three shots from my latest trip to London when, a couple of children, fascinated by a group of pigeons sheltering behind the National Gallery, spent a lively 15 minutes…