For the more than two million people who visit the National Portrait Gallery every year, this is part of the attraction. There’s something oddly comforting about gazing into the eyes of a national hero and convincing yourself he was probably a complete fraud. A mediocre plumber from the West Midlands can approach Montgomery of Alamein and, unburdened by historical knowledge, write him off as a loser, the kind of boy at school who always ended up hanging from a coat hook upside-down.