Let me strongly recommend to anyone interested in this crucial aspect of Napoleonic warfare, David Blackmore's "Destructive and Formidable: British Infantry Firepower 1642-1765" (Frontline Books/Pen and Sword 2014). The British learnt their fire discipline from the Dutch and refined it throughout the Eighteenth Century.
He uses the piece quoted by Han-Karl/Rory Muir but points out that, while not denying that the British were not quite text-book perfect at Dettingen, the 'eyewitness' from the Foot Guards was anything up to two miles from the scene he described! At Dettingen the Royal Welsh Fusiliers delivered well-maintained 'firings' that overthrew three French battalions in turn.
The key to success was relentlessly marching to within 60 paces of the enemy and delivering a rolling fire as they closed with them. There aren't too many troops who can stand up to that sort of punishment.