Sri Lanka finally gave themselves a score to defend but it still wasn't enough for their first win over England of the tour. Jason Roy made sure everyone knew they were on his manor with his second century of the series, and the second-highest score by an England batsman in ODIs, as the hosts cruised home with room to spare. These are strange times in what we must still refer to as the United Kingdom and England's limited-overs revolution is also taking some getting used to.

England players like to talk about confidence in terms of "having your tyres pumped" and Roy in this sort of form looks like a monster truck. Without his regular opening partner, Alex Hales, who was suffering from a back problem and did not bat, Roy marauded around the Kia Oval just like he has done on so many floodlit nights for Surrey. Memories of his slim returns in South Africa, where he scored 96 runs in five innings, have been subsumed by the avalanche against Sri Lanka, helping England to seal the ODIs 2-0 with one match to play.

He is currently averaging 141.00 for the series, a figure which speaks for itself in stentorian tones. In one innings, his career average rose from 33.90 to a fat, round 40.00. Roy's qualities had already been reaffirmed with his role in England's rambunctious run to the World T20 final and, at the start of the tournament, he spoke of trying not to put himself under too much pressure to "go out and whack it from ball one".

England's new ethos almost seems to demand as much from Roy, particularly as Hales often likes to take a bit more time to find his stride. But there is aggression, and there is aggression. Here, Roy was happy to putter along at a run a ball until the point at which he had made 24 off 25, then casually sauntered forwards and dumped Farveez Maharoof over long-on for the first of his three sixes. He had his moment of luck in Maharoof's next over, when a thin inside-edge missed leg stump by a whisker and flew away for four, and was soon into his pomp.

England, aside from their sloppy performance at Trent Bridge, have located gears that Sri Lanka currently cannot find and Roy drove them towards a target that initially seemed likely to be testing as if it were a summery Sunday afternoon, rather than a cold, damp night in south London. The crowd drank in each of Roy's milestones as happily as another beer from the bar but, just as he seemed set to eclipse Robin Smith's 167 - made during a 55-over game back in 1993 - he was dismissed aiming a tired swipe at Nuwan Pradeep's slower ball. His evident frustration at not finishing off the job bodes well for England.

Roy's hundred, the fastest of his three in ODIs, came off just 74 balls and a couple of years ago that would have been second only to Kevin Pietersen - a player Roy has been compared to (not least because of his Durban birthplace). It is a mark of how England's game has moved at light speed that it now sits joint-ninth in the list.

"I'm pretty happy with where I've got to," he said. "A lot of hard work has gone into it and to see the rewards is incredibly special, at my home ground, in a big game, to win a series... I'm pretty stoked."

England have kept faith in Surrey's punkish young opener and he has begun to play like a king. Sri Lanka have a prince of their own in the ranks and on another night there would have been much heralding of Kusal Mendis. England's rapid pursuit of their target made previous feats appear less dizzying but could not fully erase the impression of Mendis buckling his swash to fight the odds and the drizzle and give Sri Lankans something to smile about.

Mendis is on the dinky side but he packs a punch. Were he from South America, he would doubtless be called "la Pulga Atomica" - the Atomic Flea, as Leo Messi is sometimes known. He struck seven fours in his first 18 balls, wresting back any early initiative England thought they had won from achieving a run-out in the second over, and setting the platform for their highest total of the series.

At the start of the tour, Kumar Sangakkara had mentioned the former Schoolboy Cricket of the Year as a future star and Mendis will return home with a report card to make his family proud. His maiden Test half-century was the sole bright spot during Sri Lanka's hammering at Headingley and he added a fifty on ODI debut in Dublin a few weeks later, before following up with another against England in Bristol.

This was the first time he had pushed his score on. In his sixth ODI, it was Mendis' highest knock in any form of white-ball cricket, so he could be forgiven for not quite knowing the route to a century. But he gave the Sri Lanka innings its impetus, dragging Danuska Gunathilaka along to a first fifty of the series in his wake and allowing Dinesh Chandimal and Angelo Mathews to start shovelling coal in the engine room.

It is worth recording what a thoroughly miserable day this was on which to watch or play cricket. A sold-out Kia Oval shivered in the drizzle through the first half of Sri Lanka's innings but returned from cover when the rain finally relented in the late afternoon. This was set to the be the highest attendance for a match at the ground in modern times, after the expansion of the Peter May stand, and in more clement conditions it would almost certainly have challenged the match between England and New Zealand here last year, which aggregated 763 runs - the most for an ODI in this country.

Grey clouds scudded above the ground throughout, as the "right little, tight little island" of Thomas Dibdin's imagination lay swaddled in murky weather and the fear of treachery. Above the pavilion, a union flag fluttered above a smaller St George's cross. After Roy's feat, the rain soon returned but this "snug little island" perhaps - strangely - felt just a little more secure.

Alan Gardner is an assistant editor at ESPNcricinfo. @alanroderick