In 2013, a short royal family home-movie came to light. Dating from early October 1952, it shows Queen Elizabeth II, eight months into her reign, enjoying a family fishing expedition at Balmoral. Also prominent is the unmistakable figure of Winston Churchill, returned as prime minister a year earlier and now, a month shy of his 78th birthday, the Queen's honoured guest.
Churchill sits at the water's edge, chatting amiably to a young Prince Charles. He is relaxed but he is not off-duty. His thoughts, we now know, regularly drifted from autumnal Scotland to a barren, windswept outpost of the Commonwealth called the Montebello Islands. There, 80 miles of fthe coast of north-west Australia, Britain's first atomic bomb was about to be tested.
For Churchill, a great deal rested on the…