Dig a little deeper, though, and you soon uncover the splinter of ice concealed beneath the sunny exterior. No one wins 50-odd caps as a prop forward without mastering the tricks of the trade, from the unfathomable to the unmentionable, and for all his relaxed geniality, the 28-year-old Harlequin is now the Big Daddy of the dressing-room. When Jason speaks, the rest listen.
Strangely for a man who lists shamelessly opinionated England predecessors like Jeff Probyn, Brian Moore and John Olver among his influences, Leonard is not much interested in talking a good game. Words come cheap, after all. Yet Jack Rowell, whose phenomenal success as a club coach at Bath owed an enormous amount to clever psychology as well as tactical intuition, regards his most experienced forward as something of a closet orator.
It was to Leonard that Rowell turned during the nervous build-up to the Ireland match a fortnight ago and his choice was vindicated in the first five minutes as the England pack soaked up the early fire and fury and returned it with interest. When the team re-gathered in Marlow on Monday night to prepare for France, they were greeted once again by that chirpy London accent.
"Yes, I had a word," said Leonard before one of this week's intensive training sessions at Bisham Abbey. "What did I tell them? I told them that I wanted this side to be feared and respected throughout the rugby world. And I said that, not just because it was the sort of thing international players expect to hear in a team meeting but because I genuinely believe we have the potential to win that respect, to command that fear.
"I've been lucky enough to have played in a couple of great England sides but this one could be the best of the lot. I'm not one to brag and I have no patience with idle boasts, but I think we are capable of approaching the standards set by the All Blacks under Wayne Shelford in the late 1980s. Their rugby was as near perfect as dammit; they had experience and youth, strength and athleticism, pure animal instinct mixed with a touch of the cavalier. It was an explosive mix and I sense a bit of it about us.
"So, the point of Monday's little chat was to make sure everyone still appreciated exactly where we were trying to get to. You can't achieve perfection, but Shelford showed how close you can get. We've played two Five Nations matches and won both by 40 points without playing well. Sooner or later, we'll really click and do someone a lot of damage. Tomorrow would be ideal as far as I'm concerned."
For all his high-flown ambition, the phrase "feet on the ground" might have been coined for Leonard. A Quin he may be, but there is nothing of the Eton and Cambridge in his background - mention the name Virgil to him and he is more likely to think of Thunderbird 2 than the Aeneid - and the England dressing-room is a better place for his brand of earthy realism. In an age of professional craftmanship, the silver spoon counts for less than nothing.
Rather like Jeremy Guscott, a fellow comprehensive school type, Leonard climbed through the ranks on the back of his own raw talent to establish himself as a major player on the world stage. Today, he goes toe to toe with Christian Califano, the soldier from Toulouse, and there are many aficionados of the front-row jungle who have the two men level pegging at the very top of the tree. "Good boy, Califano," he says with one of his soft, reflective smiles. "I'm looking forward to seeing him again."
That carefree line speaks volumes for Leonard's seemingly inexhaustible enthusiasm for life amid the bump and grind. The youngest Englishman to reach a half century of appearances, he has missed only one match - the 1995 World Cup pool game with Western Samoa, when Rowell fielded a shadow side - since making his debut in Buenos Aires in 1990. When you consider that he suffered a ruptured disc in his neck in 1992 - the worst imaginable injury for a prop - his durability appears more impressive still.
"One of the great joys of being involved at the top level is that you take something with you from every game. No matter how much you think you know, how many times you've been in the situation before, you never go through an international without learning some sort of lesson.
"When I captained England against Argentina in Phil de Glanville's absence just before Christmas, I learned one of the most valuable lessons of all - namely, never to assume anything about your opposition. We'd looked at the Pumas on video and at no point did we see the ball move beyond the outside-half, so we went into the game expecting more of the same. Of course, they caught us on the hop by playing with width as well as passion and as a result, things began to go wrong.
"But what people tend to forget about that game is the result on the old scoreboard at the end. When it came down to brass tacks, we had enough discipline and enough belief to squeeze out a win. As a performance, it was fairly disappointing; as a victory, it was crucial to our confidence."
Leonard believes the feel-good factor among the forwards, in particular, will benefit from Rowell's commitment to continuity. "The really strong English packs of recent vintage built up their togetherness over a period of 20 or 30 internationals and I want to see this unit, which I believe possesses world-class potential, given a similar opportunity," he said.
That sort of timescale would give the boy from Barking close on 90 caps. What price the century?Reuse content