Well whenever people needed to communicate accurately Colin.
You see in the days before trains, everybody set their clocks by the sun, but trains needed a universal time. And once people started travelling in them a standard form of English - even if learnt apart from the local dialect - became necessary to ensure communicability.
And so amongst educated people a standard grammar and to some extent a standard pronunciation occurred.
Everybody who was educated could read and write standard English and talk to each other.
In the last ten years I have three times had to give up on a support call simply because I *could not understand* the person on the other end. One was the thickest Subcontinental accent I have ever heard although I think it was grammatically correct, another sounded like a slurred mumble of Pidgin English/Estuarine elision. But give me a Welsh person or a Glaswegian or a Brummy or a Geordie, and I am perfectly happy as long as they have been taught standard English, the accent doesn't matter.