Inside St Paul's, the epitaph of its architect offers a single instruction: if you seek his monument, look around. Three centuries on, the cathedral takes its own advice. Caught between the glass walls of a newer London, Christopher Wren's dome turns to find itself waiting on either side, doubled and tripled in the panes, gold against a tide of blue. The new city tried to rise above it and only managed to reflect it. Some monuments do not need defending. They simply turn their heads, and discover the whole skyline has learned their shape.