Jones, 56, says deciding to go back into full-time coaching with Japan (his previous role in Hong Kong involved “80 per cent coaching development and 20 per cent coaching the team”) was “probably a brave move or a silly one depending on how you look at it”, but there’s no question it turned out well for all involved.

“There were two attractions, one was the World Cup itself, one was working with Eddie,” he says, as he paints a picture of an absolute obsessive.

“A lot of people had told me he’s a very difficult guy to work for. And everything I’d heard was true – he’s a tough guy to work for.

“He’s workaholic, probably gets by on four or five hours’ sleep a day, and he expects his staff to do the same. I’d never worked so hard since I left industry 20 years ago. We would start at six in the morning and be pretty much full-on ’til eight, nine, 10 at night ... but it was so informative working for a guy of that calibre.”

Jones self-deprecatingly describes himself as “a daft old Welshman running the entertainments committee” in Japan’s back-room, though there’s no doubt that his efforts were key. French set-piece expert Marc del Maso, forwards coach Steve Borthwick (with Eddie Jones again now and “a future England coach” according to Leigh Jones) and Japanese assistant coach Keisuke Sawaki made for some interesting team meetings, but the message got across.

“We’d have two translators, one from Japanese to English, another from Japanese to French and if you walked in to a team meeting you’d have thought it was absolute mayhem, but somehow it worked,” says Jones.

“Me and Eddie are about the same age and both like a glass of wine and I became almost the shop steward at times for the rest of the coaches. I’d have to go broker deals – ‘Listen Eddie, you’re pushing a bit too hard, we need to relax’ – but he’s just relentless, absolutely relentless, and that’s obviously why he’s been such a success.”

As well as that work ethic, key to the team’s success was in-depth statistical and analytical insights into every aspect of the game.

“Without any doubt the Japanese team were the fittest team I’ve ever worked with,” says Jones. “I’ve never seen a group of players pushed so hard, and almost questioned Eddie at times if he was pushing them too hard.

“I’ve also never been involved in such an intellectual rugby environment where everything was discussed, analysed at the critical level, measured and then reanalysed; it was a really sophisticated environment.”

“Again that was driven by Eddie and probably one of best conditioners in the world, John Prior, he and Eddie had worked for a long time, scrutinising, planning and basing everything from scientific measurement. Eddie’s well-renowned for being a stats guy and if your stats are telling you you’re doing the right thing great, if they’re not Eddie would take a different direction, so it was based as much as rugby can be on scientific underpinnings.”

As an insight into Eddie Jones’s obsessive nature and canny man-management, Leigh recalls the aftermath of the South Africa win – not a wild celebration but instead a late-night dressing-down.

“You can imagine the avalanche of euphoria ... [but] after the win we were sat in a meeting at 10.30pm discussing critically why one aspect of our game hadn’t worked particulary well and making sure we put it right for four days’ time against Scotland.

“I talked to Eddie afterwards, said, ‘What was that about, that’s a bit of overkill.’ He said, ‘Mate, I had to get you boys back on the floor, because if I don’t get you on the floor, you’ll never get the players on the floor.’

“So this is the brilliance of the guy – next morning we were all quite grounded as a group of coaches so our job then was to ground the players and look forward to Scotland.”

Hong Kong made the repechage stage of qualification for the last World Cup, beaten by Uruguay who went on to qualify. That has boosted the HKRU’s belief that they can emulate Japan and reach a World Cup, and Jones now aims to pass on the insights he learned.

“I was part of probably the best rugby programme in the world over the last two years, so there’s an awful lot of information I can bring back to Hong Kong from that,” he says. “I know where we have to get to, the challenge for us in Hong Kong is taking steps to get there and how quickly we can take those steps.

“Even if we don’t qualify for the World Cup we have to stay on the tails of sides like Japan and Korea. Japan have gone into the Super Rugby competition this year with a view of planning for the World Cup and I’ve heard Korea with Hyundai are taking a professional team to the Japan league so if Hong Kong don’t follow suit in a relative way we’re going to be left behind.

“My job fundamentally is taking best 30 players in Hong Kong and trying to put them on a professional programme; the challenge then is to be getting enough games for them so they’re better prepared for the World Cup and to give it the best shot we can.

“Can we make up the ground that exists between us and Japan, I don’t know ... [but] we’re not that far away and if we can get the right group, right preparation behind us, it’s doable.

“If you give a group of men a cause they can believe in, what they can produce continues to astound me. When you get a top quality coach like Eddie and the time he was granted with players to get them fit and strong it’s a simple recipe for success that’s going to serve us well in Hong Kong.

“It was a delight and honour to be part of that programme and see how it worked.”

Meanwhile, as Eddie Jones plots his Six Nations campaign, Leigh says England can become one of the best in the world again – if they listen.

“It will be an interesting challenge – he drove the Japan players harder than I’ve ever seen a group of players driven before. He’s going to need buy-in [and] I’m not sure western players have the same work ethic. He’d expect that work ethic – whether he gets it or not remains to be seen.

“If he does get that buy-in from the players, England will be a real force to be reckoned with. If he doesn’t get that buy-in then it’s a whole different story, so it’s kind of watch-this-space for me to see if he does get the backing of the players.”