Kensington Palace Gardens has a good claim to be the most exclusive road in Britain. It runs alongside the palace that was once home to Diana, Princess of Wales, and is now the residence of her son, Prince William, and assorted other royals. Several billionaires are reputed to own mansions on the street — including Roman Abramovich, the Russian owner of Chelsea Football Club and Lakshmi Mittal, the Indian steel tycoon. National flags flutter from many of the other houses to signal that they are ambassadorial residences. It is the Japanese ambassador — who resides at number 23 — who I am visiting on a bright summer’s afternoon.

Despite its rich and powerful residents, the wide avenue — which runs from Notting Hill Gate to Kensington — is only semi-private. There are police boxes at both entrances and armed policemen, still a relative rarity in London, can be seen strolling up and down. But nobody asks me my business, as I turn into the avenue from the Bayswater Road and stroll past a few joggers and pedestrians. At the appointed hour (midday), I press the bell on the gate of the Japanese residence. After a few moments of silence, the iron gates swing open and I walk up the crescent-shaped drive. The front door also opens, as if by magic, and I am ushered left, into a comfortably appointed sitting room. Sitting on a well-stuffed cream-coloured sofa is Koji Tsuruoka.

The ambassador is a relative newcomer to London. He arrived in June, after spending three arduous years negotiating the Trans-Pacific Partnership, a trade deal that is meant to break down economic barriers between the US, Japan and 10 other Pacific nations. As we are served tea, I suggest to Tsuruoka that the role of ambassador in London probably offers a gentler pace than the intense and highly technical TPP negotiations.