I arrive at Brasserie Zédel just in time for a 1pm lunch to find Sir Stephen Wall already perched on a banquette at one of the restaurant’s closely-packed tables. Sir Stephen, a former diplomat, greets me and says: “Oh, thank goodness, you are not wearing a tie. I’ll take mine off as well. I was only wearing it in your honour.” He removes his bright red tie and sits facing me, in a white shirt and blue blazer. “Ties are so incredibly uncomfortable, aren’t they?” he says and we chat about why, nonetheless, men insist on wearing them. “When I was working, it was the uniform,” he says. “And I wore the uniform.”

Casting off stifling uniforms is something of a theme of Sir Stephen’s life. For most of his career, he was a high-flying diplomat — culminating in a spell as Britain’s ambassador to the European Union in the 1990s and then as Tony Blair’s chief adviser on Europe. After retiring from the civil service in 2004, he became the senior policy adviser to Cormac Murphy-O’Connor, who was then Catholic archbishop of Westminster. But, in his early sixties, Sir Stephen decided that he no longer believed in God. Around the same time, he told his wife and adult son that he was gay. He is now a prominent advocate for gay rights and, in his own words, “a recovering Catholic”. In retirement, he has also shed his diplomatic neutrality and become a vocal advocate of continued British membership of the EU.

Taken together, this all sounds like a major mid-life crisis. But the man sitting opposite me seems, physically and temperamentally, little changed from the calm diplomat I first met in Downing Street in 2000, when he briefed me about Britain’s negotiating position ahead of an EU summit in Nice. Now a young-looking 68, with closely-cropped grey hair and the same professorial spectacles, he remains the consummate mandarin — quick-witted and able to assess every angle of a question rapidly and with dry humour. Yet, behind the civil-service reserve, there was always more than met the eye to Sir Stephen Wall. Tony Blair wrote in his memoir, “Stephen was very professional and proficient?.?.?.?but underneath you could tell he was a riot of strong emotions, opinions and insights.”