Credibility

As an aspiring freelance journalist of sorts, I find the subject of credibility to be of great importance, and being a student of philosophy, I cant help but find myself philosophising about it.

What is credibility? What does it actually mean?

credible
ˈkrɛdɪb(ə)l/
adjective adjective: credible
1. able to be believed; convincing.
"few people found his story credible" synonyms: acceptable, trustworthy, reliable, dependable, sure, good, valid; More feasible, viable, tenable, sustainable, maintainable
"the existing lists did not form a credible basis for free and fair elections" antonyms: untrustworthy capable of persuading people that something will happen or be successful.
"a credible threat" synonyms: believable, plausible, able to hold water, within the bounds of possibility, reasonable, sound, compelling, persuasive; More raresuasive, assuasive, verisimilar, colourable, cogitable
"very few people found his story credible" antonyms: unbelievable
Here is Google definition of the word credible, we can see here that credibility encompasses a plethora of positive sentiments and motives. Truth, viability and compulsion to name just a few. So what does this tell us about the requirements for one to be deemed credible? It tells us that in order to believe something, we must be given no cause for reasonable doubt. We have to be able to trust the source. In order for this to be so, the source should be impartial, of sound mind and without any conflict of interests.
To contextualise this for you, I conducted a little experiment. Currently in the United Kingdom, we have a bit of a problem with the upper echelons of our society, no, its nothing to do with taxes (but good guess!). The British establishment is currently undergoing an internal investigation into allegations of institutionalised child abuse, spanning over three decades. An inquiry into an institutionalised crime, being conducted by the institution? This doesn't exactly give off an air of “credibility” now does it? After all, you wouldn't let an accused man conduct his own trial, that would be what we call a conflict of interests.
So in order to find out just how travesty's like this are able to go unchallenged, I decided to see what my fellow man thought of it, whilst toying with the idea of credibility. I opened a debate on a UK political debate forum, using this article in my original post.
Instantly the subject of the articles credibility was subject to question, on the grounds of the news source being unreliable. Fair enough. Next came this article, an independent publication, yet from a respected and generally seen to be “credible” source, within the field of journalism. This too was met with criticism.
The debate following this brought to light the dossier Geoffrey Dickens compiled in 1983, and handed on to Leon Brittan. Alarmingly, the existence of such a dossier was brought into question, even though the we know for a fact that the dossier existed, after discovery of a letter from Brittan to Dickens acknowledging receipt of the document. In order to educate my fellow debaters on the facts of the allegations, I provided a Wikipedia link to a page dedicated to the Geoffrey's dossier. Can you guessed what happened next? Bingo, Wikipedia's credibility as a source was gunned down.

All the while this was happening, I was being assured that none only did factual occurrences like Brittan's admittance to receiving the dossier (thus legitimising its existence in terms of credibility) never actually happen, but that the mere notion of institutionalised child abuse was tin foil hat level hokum. Not a single person involved in that discussion (other than myself) was able to provide a single shred of information, history or journalism to support their argument. They're credibility was coming from some seemingly unknown source, or maybe it was hiding in plain sight? Of course! They wanted mainstream sources of information.
Scalded from unrelenting flaming,