As the Singaporean prime minister settles into his seat for lunch, I am fussing with my tape machines – two of them, just in case one fails. Lee Hsien Loong smiles faintly and says: “The NSA will give you a copy.”

It is an unexpectedly subversive remark from a man I had expected to be the epitome of earnestness. The prime minister has a reputation as a cerebral technocrat, without a frivolous bone in his body. He even looks austere – tall, slim, grey hair and dressed in a dark suit and tie. So the biggest surprise, during our lunch, is how often Lee laughs. Over the course of the next hour, a variety of grim subjects provokes an incongruous chuckle or a broad smile – the Japanese occupation of Singapore in the second world war, the west’s mishandling of the revolution in Ukraine, China’s fear of separatist movements and the bankruptcy of Iceland. It is not, I conclude, that the Singaporean prime minister is a callous man. It is just that his way of taking the edge off the most difficult topics is to laugh while discussing them.

We meet at 11am in the Park Terrace of the Royal Garden Hotel in London. It is early for lunch but this is the hour his staff have carved out between other events on his visit to Europe: a nuclear security summit in the Netherlands, speeches, meetings with businessmen and Britain’s prime minister David Cameron, and, later that day, an event to celebrate Singapore Day in a park in east London, which 10,000 expats have registered for. With typical Singaporean thoroughness, the PM’s staff had emailed me the restaurant’s menu some days before our meeting and taken my order. We are positioned at a corner table, overlooking Kensington Gardens, with the sun streaming through the windows. Our first course – salmon and crab terrine – is brought promptly. Given the early hour, we both stick with water – although Lee, slim and fit-looking, does not strike me as a likely boozer.