Prince Harry’s closest circle could be set to join him in a luxury mountain retreat to celebrate his 40th Birthday.The Duke of Sussex plans to celebrate his 40th birthday on Sunday with his family in California, before heading off on a break with a group of his closest friends. On the year he turned 30 in 2014, the year after returning from his second tour with the British army in Afghanistan, he formed the Invictus Games. The Duke of Sussex is expected to go to a mountain retreat in California with his ‘closest mates’ – some of whom may fly over from the UK. One of those almost certain to be …