“Used to standing quietly in the shadows while the spotlight shines on a Leonardo or Caravaggio or Velázquez, the National Gallery’s warders – or assistants, as they are known these days – do not tend to draw attention to themselves. But, at the moment, that is exactly what they are doing. Last week’s two-hour stoppage, which saw between 30 and 40 assistants walk off the job, forced the temporary closure of around 35 rooms, though not the blockbuster Leonardo exhibition.”