"It is old news and we freepers are ridiculous to think that someone copying notes, and destroying the originals is something to stupid to consider..."
It certainly is old news, as evidenced by the link someone provided above where the Foreign Office confirms that one of the documents is genuine - that is dated September of last year.
Consider the journalists methods all you want, but in cases like this the simplest explanation is probably the correct one. We know that a lot of things have been leaked that are known to be genuine (Lord Goldsmiths legal advice for example). We know that this particular journalist has received leaked documents that are confirmed genuine. We know that no-one from the British government has disputed that the documents reported are what they say they are (they dispute the interpretation placed on them, but that's not the same thing at all). Simplest explanation is probably the right one.
The reporter's story that he 'copied' the documents and destroyed 'originals' to intentionally sunder the chain of custody makes any such documents worthless on their face, prima facie. Don't believe me? Have your hero try to introduce them into any US court (other than Kangaroo Konyers and whoever he can fit into the water closet with him) as evidence.
So you are wasting your time here, noob, unless of course someone is paying you, in which case you are wasting their time.