I visited Bangkok expecting to eat a lot of Thai food, which I don’t think was unreasonable in the circumstances. So it was surprising that the most memorable meal of the trip turned out to be Japanese. Bangkok has one of the largest Japanese populations in Asia outside Japan. The official figures are about 50,000 but most estimates put it at at least double that, when you consider the numbers on long-term tourist visas who neglect to mention the fact to the authorities. About a quarter of work permits issued are to the Japanese.

The relationship between the two countries existed well before the Japanese occupation of Thailand during the second world war. In the 17th century, the royal palace of the Ayutthaya Kingdom, which predated modern Thailand, employed samurai guards, and the two nations traded more or less happily.

Now many of the largest Japanese firms have offices here, and there’s no shortage of businesses to make them feel at home. There are hundreds of Japanese restaurants, Japanese drinking dens, Japanese supermarkets, Japanese karaoke bars, Japanese massage parlours (legitimate) and Japanese massage parlours (alternative). There is a Thai-Japanese school, with nearly 3,000 Japanese pupils, open only to Japanese nationals. There are Japanese dry cleaners, and apartment blocks rented exclusively to Japanese people. The vast markets at Paragon and Emporium have dedicated Japanese sections.

Bangkok’s Sukhumvit Road at night. The area around it sees the largest concentration of the city’s Japanese population. Photograph: Manjik photography/Alamy

The population is concentrated around Sukhumvit, the district in the centre of the city surrounding Sukhumvit Road, the arterial that bisects Bangkok and runs off east towards the border with Cambodia. The streets off it, called soi, are numbered in ascending order from west to east. It’s a smart area, popular with expats from all over the world. You might have stayed there if you were visiting Bangkok on business, but you probably wouldn’t have found yourself there on a backpack-laden week drinking buckets on the Khao San Road.

Bangkok is a five-hour flight from Tokyo, near enough that the best fish can be flown from the Tsukiji fish market

It was in an alley behind Soi 23, at Sushi Juban, an omakase restaurant where the chef chooses your order on your behalf, that I had the best meal of my trip. My girlfriend and I were meeting Lawrence Osborne, a British novelist who has made his home in Bangkok. He warned us that the restaurant was hard to find, and he would wait for us under a more obvious landmark, a luminous yellow sign for a testosterone replacement clinic.

Away from the chaos of the main road, the streets are narrow and loomed over by arching trees, high walls and the condominiums behind them. Without guidance it would be easy to mistake the quietness around here for an absence of community. The Japanese are as fond of keeping their own counsel here as anywhere else.

Sushi Juban, an omakase restaurant at Soi 23, where Ed Cumming had the best meal of his trip

But there is plenty going on. Aside from Juban, for a more fusion-driven list there’s Isao on Soi 33. Or Kisso, a laid-back spot on Soi 19. On Soi 39, Don Don does excellent noodles. Or for top-end sake, Ten-Sui, a little further west on Soi 16. Bangkok is a five-hour flight from Tokyo, near enough that the best fish can be flown from the Tsukiji fish market. Tuesdays and Fridays are apparently the best days for this.

Much of the Japanese culture in Bangkok is like this: hidden from the view of tourists.

Back at Sushi Juban, a few other diners sat facing each other. Over the next two hours, a chef assembled a series of dishes and passed them over the counter. There was plenty of nigiri, little Lego bricks of rice topped with gleaming salmon, fat tuna belly, sweet smoky eel, octopus. Crunchy tempura prawns, long as pencils, came straight from the fryer, defying us to burn our tongues. It was all perfect.

After dinner we headed to Hailiang, a tiny Japanese whisky bar 10 blocks down Sukhumvit from Juban, with 12 seats and low jazz on the speakers. Behind the bar stood Jay, who comes from Osaka but speaks enough English to guide customers through an exhaustive list of Japanese and Scottish single malts, blends and bourbons.

In international rankings, the Japanese now have the edge over the Scots. Some of Jay’s selections cost almost $100 a shot. After a couple of rounds, he presented each of us with a tiny brick of warm bread, on top of which a curl of salted French butter was just starting to melt.

“For the Japanese we serve rice, but westerners prefer the bread,” he explained. Down the counter, a Japanese man slipped off his stool.

Hamachi being prepared at Jua restaurant, ‘a western twist on an izakaya – a type of informal Japanese pub – on the site of an old speakeasy casino’

Much of the Japanese culture in Bangkok is like this: hidden from the view of tourists, available most easily to those in the know. But not all of it. Away from Sukhumvit, in a revitalised area by the river, two Americans, Jason Long and Chet Atkins, have set up Jua, a western twist on an izakaya – a type of informal Japanese pub – on the site of an old speakeasy casino. Jua is Thai slang for “hit me”, a testament to the blackjack-style game that used to be played here. Chet is the chef, and has cooked Japanese food all over the world. Jason is a photographer, who got the idea for the bar on a job shooting portraits of sake distillers. Large prints of his work line the walls behind the bar.

“There’s a huge Japanese community in Bangkok, so there’s always been incredible Japanese food,” Jason explained. “But at most of the places, when you walk in, it feels like you’re in Osaka or Tokyo. We wanted to do something more western, especially since it’s two white guys doing Japanese food – it didn’t feel right to do it in that style.” He poured us a glass of yuzu sake. It was tart and cold, and against the vegetal warmth of the Bangkok night, perfect.

Way to go

Eva Air flies from Heathrow to Bangkok from £676 return. The Eugenia Hotel, Soi 31, Sukhumvit Road, has doubles from £94