World Cup - England win to break Scottish hearts
The Scots had the better of the first-half and took a 9-3 advantage into the interval.
Scotland added another penalty after the break, which meant they briefly had the eight-point advantage they realistically needed to progress to the knockout stages.
But a drop goal and penalty from Jonny Wilkinson drew England close, and Ashton crossed with just three minutes remaining to deny Scotland the consolation of a win.
The game was dominated by the boot - perhaps no surprise when Scotland's original game plan had to be quickly revised after Ruaridh Jackson limped off with a hamstring pull just five minutes in.
Dan Parks came on for the visibly distraught Jackson, and delivered a masterful first-half kicking display as the Scots made the better start.
Chris Paterson's ninth-minute penalty from the touchline gave them an early advantage, one which they extended seven minutes later when Parks's penalty crept over the crossbar.
England racked up several penalty chances of their own in the first half, but Wilkinson's boot let him down. He missed three consecutive chances before finally closing the gap 33 minutes in.
The men in navy blue ended the half on the front foot, and Parks restored the six-point advantage with an authoritative drop goal.
Scotland coach Andy Robinson was much the happier of the coaches at the break, but Martin Johnson's men emerged with more purpose in the second half.
A strong 10-minute spell began with a thumping tackle from Manu Tuilagi and Delon Armitage surging down the touchline in England's most effective moment of the match up to that point.
England could not make an impression on the scoresheet, however, with the erratic Wilkinson missing a drop-goal attempt from close range.
Scotland weathered the storm and fashioned a chance of their own - a scintillating move culminating in Simon Danielli's chipped kick beyond the try line which a full-stretch Ben Foden only just beat him to.
Paterson's pinpoint penalty from the touchline gave Scotland a tantalising taste of a famous upset.
But their lead lasted just a minute as England scored on their next foray into the Scotland half.
Wilkinson, having blown a simple drop-goal opportunity minutes beforehand, sent a superb kick through the posts from further out and on his right foot to reduce the deficit.
And he sent his next penalty chance sailing between the sticks to bring England within three points once more.
Scotland were running out of opportunities to find a sufficient winning margin as the game grew scrappier under the weight of numerous substitutions for both sides. A limping Mike Tindall and Wilkinson himself, clutching his arm in pain, were amongst the withdrawals.
It was a combination of replacements who secured the victory - Richard Wigglesworth's quick ball to Toby Flood gave the fly-half room to throw a looping long ball right to Ashton, who slid in at the far corner. Flood added the conversion from an acute angle.
England went ahead for the first time in the match, and for the second week in a row a committed Scotland performance ended in narrow and late defeat.
9' Paterson penalty - England 0-3
16' Parks penalty - England 0-6
33' Wilkinson penalty - England 3-6
40' Parks drop goal - England 3-9
56' Paterson penalty - England 3-12
57' Wilkinson drop goal - England 6-12
62' Wilkinson penalty - England 9-12
78' Ashton try - England 14-12
78' Flood conversion - England 16-12