A very well respected early medieval historian Guy Halsall wrote an excellent book that you should run out RIGHT NOW and purchase, called "Worlds of Arthur."

The short and historical answer is, "we will never know." And... that is all the historical record has to say on the subject. And this is actually being generous.

If you know anything about what people say about the historical King Arthur, you know the contemporary source, Gildas, doesn't mention Arthur at all, and the modern histories/pseudohistories tend to jump through quite a bit of contortions in order to convert a non-mention into a "argument from silence."

Arthur's first mentions are in the imprecisely dated, Y Gododdin, which could theoretically have been written anywhere between the 7th and the 11th century which is quite a wide range, and still more than 150 years after a presumed real Arthur at its earliest. Even then, in Y Gododdin, all that's said is "He fed black ravens on the rampart of a fortress, Though he was no Arthur." That's literally it. All we know is that "an" Arthur existed, and that the subject of Y Gododdin was "not him."

Guy Halsall gives a far more detailed breakdown of the first source to describe Arthur in detail, Nennius, but I'll summarize the results of his analysis, which were that it was written in a "mirroring" stylistic device that lends itself more to the creation of a myth than the description of an actual history. His explanation is actually far more detailed than this, but it has many moving parts, and its worth getting the book instead.

Barring the deus ex machina discovery of a massive cache of previously unknown texts that can be precisely dated to contemporary Arthurian times that talks explicitly about Arthur, and whose details closely match our expectations of him, the "we will never know" will likely stand.

tl;dr - We'll never know.

Source:

• Halsall, Guy. Worlds of Arthur: Facts & Fictions of the Dark Ages. Oxford: Oxford UP, 2013. Print.

Can someone elaborate/summarise how we went from a single reference 'he was no arthur', to the full tale of Camelot, and Merlin and all that.

What were the earliest references to the various parts of the story. We're they added in at different times?

In the various sources cited are there other figures mentioned extant from the same time as the supposed Arthur whose existence can be historically verified ?