England 309 for 4 (Roy 162, Root 65) beat Sri Lanka 305 for 5 (Mendis 77, Mathews 67*, Chandimal 63, Gunathilaka 62) by six wickets
Scorecard and ball-by-ball details
Jason Roy lit up a gloomy evening in South London with a stunning matchwinning 162 - the second highest ODI score ever made by an England batsman - as Sri Lanka's best batting performance of the series was handed an inferiority complex by a relentless display of hard, clean hitting, allied to the sort of sharp running between the wickets that their own players couldn't come close to emulating.
Chasing a stiff Duckworth-Lewis-adjusted target of 308 in 42 overs, following a two-and-a-half hour break for rain that had robbed Sri Lanka's own innings of vital momentum, Roy crashed 13 fours and three sixes in the course of a 118-ball onslaught to put the result beyond any doubt.
Roy's first hundred came up from 74 balls, his 150 from 109, and though he eventually fell with 27 runs still needed, bowled by a Nuwan Pradeep slower ball with Robin Smith's 23-year England record of 167 not out at his mercy, Jonny Bairstow and Jos Buttler were on hand to seal victory with a cool 11 balls and six wickets to spare. There was even time, before the fun was concluded, for Farveez Maharoof to revive the Buttler mankading saga from the same series two years ago, as he stopped in his run-up to serve England's non-striker a pointed warning for backing up too far.
The extent of England's dominance evoked memories of a similar run-chase that Sri Lanka themselves pulled off at Headingley exactly ten years ago this week, and which Sky Sports presciently chose to replay during the long rain delay. The footage revealed a bygone era in which England's cricketers were a white-ball joke and Upul Tharanga, Sri Lanka's puzzlingly used No.8 batsman, was a thrusting No.1 who claimed a share in matchwinning 286-run opening stand.
But in spite of that long and tedious hiatus - there had been enough precipitation in and around London to cause the final day of Middlesex's Championship match at Lord's to be a complete wash-out - it seemed the rain-gods south of the river were happy in the end to kick back and watch a masterclass from a batsman who had scored 279 runs from 220 balls since Sri Lanka last managed to prise him from the crease, in the tied first match of the series at Trent Bridge.
Following the early loss of Moeen Ali, who was promoted to open after Alex Hales suffered a back spasm while fielding, Roy added 149 for the second wicket with Joe Root, whose 65 from from 54 balls was a timely reminder - after a string of single-figure scores - that failing to spend much time at the crease isn't always the same thing as being out of form.
Eoin Morgan, by contrast, proved the wisdom of an alternative adage - one pertaining to Sod's Law - when, having ground his way to 22 from 27 balls including a trademark lofted six to hint at a restorative innings, he was sawn off by the most stunning catch of the summer so far, a one-handed swallow dive from Danushka Gamuthilaka at backward point to intercept a Suranga Lakmal long-hop.
Roy, however, needed no such luck, good or bad. The power and placement of his strokeplay was in evidence from his third delivery, when he latched onto a nugget of width from Lakmal to drill the first of his fours through the covers. This was his second hundred in as many completed matches and his first on his home ground at the Kia Oval, and that combination of peak form and utter familiarity with his surroundings meant that Sri Lanka's attack never had a prayer.
Striking the ball with high hands that enable him to impart power and speed into even the merest of blocks, and with fast feet that rarely fail to provide him a stable base from which to climb into his strokes, Roy turned the heat on a diet of toiling bowlers whose lengths were consistently too short for a pitch so true. Each of Roy's three sixes were emphatic stamps of class - full-blooded golf swings into the stands at long-on - as the series was sealed with a game, at Cardiff on Saturday, to spare.
The back of the run-chase was broken by Roy's second-wicket stand with Root, whose ability to think on his feet was evident in a subtle change of strategy, particularly against the medium-pace of Angelo Mathews. Having been bowled by a Mathews inducker for 2 in his only previous dismissal of the series, Root resolved to hang back in his crease and play the ball strictly under his eyeline, and reaped the rewards with a typically stealthy knock, packed with cheeky boundaries behind square as he reached his fifty from a brisk but never hurried 37 balls.
The ease of England's victory arguably did a disservice to the combative efforts of Sri Lanka's own batsman, particularly Kusal Mendis and Gunathilaka, whose 128-run stand for the second wicket was Sri Lanka's highest partnership of the series.
Mendis, who has been Sri Lanka's most promising young batsman of a tough campaign, top-scored with 77 from 64 balls, his fourth and highest half-century of the tour and his most fluent yet. His application could, and probably should, have been even more handsomely rewarded, however, had it not been for the onset of rain, after one ball of the 19th over, which robbed his innings of all momentum.
Five balls after the resumption, his attempt to take on Rashid's wiles resulted in an open-faced miscue to a diving Liam Plunkett in the covers, and soon afterwards, Gunathilaka was gone as well. Displaying the skills he picked up at the Big Bash and honed during England's World T20 campaign, Rashid used his googly to fine effect in the damp weather, and appeared to sucker his prey with the one that went the other way, as Gunathilaka made room for a lofted drive over the covers, but instead chipped a cramped drive to Moeen at wide mid-off.
England were a more focused unit in the second half of their bowling effort, although they struggled once again to separate Sri Lanka's old guard of Mathews and, in particular, Dinesh Chandimal, who followed his 62 from 77 balls at Bristol with a sparkier innings of 63 from 51.
Mathews, passed fit despite his ongoing hamstring issues but never looking fully mobile, had been the silent partner for much of the second half of Sri Lanka's innings, but nevertheless finished the innings in style, bringing up his fifty from 47 balls with an arrow-straight drive off a near-perfect Plunkett yorker, then celebrating with two more fours in consecutive balls. His efforts carried his side past the 300-barrier, but any thoughts of psychology coming into play were quickly scotched when Roy got into his stride. He is in a rare vein of form, and England are a team going places on his watch.
Andrew Miller is UK editor of ESPNcricinfo. He tweets @miller_cricket