Of all the causes dear to Prince Harry’s heart, few matter more than Sentebale, the charity he co-founded as a carefree bachelor back in 2006 which dedicates itself to tackling poverty and AIDS in Lesotho and Botswana. But from now on its work will continue without one of its shrewdest operators, advertising king Johnny Hornby.

I can disclose that Hornby, 56 – half-brother of About A Boy author Nick Hornby and brother-in-law of bestselling thriller-writer Robert Harris – has stood down after 11 years as a trustee, during the last five of which he was chairman of Sentebale’s board.

His departure has prompted Harry to pay tribute to his ‘leadership and vision’ and ‘dedication, partnership and friendship’. Describing Hornby’s commitment to the children and young people of southern Africa as ‘incredible’, Harry and the charity’s co-founder, Prince Seeiso of Lesotho, thank him for enabling Sentebale to ‘deliver holistic support to thousands in Lesotho and Botswana’.

Fellow entrepreneurs in cut-throat ad land acknowledge Hornby’s talents. ‘He’s smart, no doubt about it. A smooth operator and a hard commercial man,’ one tells me, adding that Hornby is in many respects comparable to advertising titan – and his one-time ally – Sir Martin Sorrell.

There’s certainly no doubting Hornby’s smoothness. During his time as a trustee at Sentebale, he was regularly snapped in easy, amicable conversation with Harry and Meghan, or walking discreetly behind them, as he did when the couple attended a London performance of Hamilton, the award-winning rap musical.

Yet his intimacy with the Sussexes didn’t preclude him from replicating a similarly warm friendship and professional relationship with someone very far removed from the couple’s circle – Who Wants To Be A Millionaire? presenter Jeremy Clarkson.

Sharing the high-octane broadcaster’s ambition and optimism, Hornby joined him in a brewing venture, Hawkstone, to produce a potent lager, as well as a pale ale and a cider.

Their partnership might have gone unremarked upon, had it not been for Clarkson’s decision to write an article, late last year, in which he pointed out that he hated Meghan on ‘a cellular level’ and added – seemingly inspired by a scene from Game of Thrones – that he wanted to see her paraded through the streets, naked, while people threw excrement at her.

When this provoked howls of criticism, Clarkson wrote a private, confidential letter to Harry on Christmas Day.

It was then, it is said, that Hornby stepped in. ‘Johnny is friends with both Jeremy and Harry, so he was kind of in the middle of it all,’ one of Hornby’s allies said. ‘He suggested that it would be a good idea for Jeremy to apologise.’

Clarkson evidently took the hint and wrote a full and abject public apology.

It failed to placate Harry and Meghan, who issued a statement, stating that it was ‘clear that this is not an isolated incident shared in haste, but rather a series of articles shared in hate’.

But the apology did work in one respect: a boycott of Hawkstone lager, which seemed possible – and which had already been instituted by one pub – did not materialise.

Only the most cynical would suggest that Hornby’s sole motivation was to protect his shared brewing interests with Clarkson. When asked in the past about his work for Sentebale – and whether it might be a stepping stone to a knighthood – he immediately replied: ‘If anyone deserves one it would be Harry himself for his work on it.’

But the end of his association with the charity does perhaps make things a little easier for his second wife, Clare, with whom he lives in Notting Hill, West London. Not one to be overshadowed, she founded and runs clothing brand Me+Em’s, valued at well over £100million.

Asked last year who had the biggest impact on her business, she unhesitatingly replied ‘the Duchess’ – not Meghan, but Catherine, who, at the time, was, of course, the Duchess of Cambridge. ‘There isn’t anyone like her. In terms of global appeal, nothing comes close,’ added Clare. ‘And quite rightly. She is amazing. So stoic.’

Nothing that even her supremely smooth husband could do about that…