Can I beg people, for all that is decent, not to click on Daily Mail 'exclusives'. And certainly not to give a second's credence to a single word written.
Regardless of what anyone thinks about the Daily Mail (and it's far from my favourite news source) it's still worth remembering that in the U.K. they are subject to the laws of liable.
In other words, they can't just print anything they wish without being subject to legal action.
They also can't make false representation in the sense if they say they have a source that has provided information even if they are not named they should have verified that person is who they claim to be and that the information given is likely to be accurate in so far that person would have had access to it.
Having said that most credible news organisations would never base a story on a single unnamed source - they would want to see corroboration from multiple sources expect in very unusual circumstances or where other evidence (eg documents) backs up the claims of a single source.
So whilst I'm not holding up the Mail as anywhere near a paragon of journalistic/editorial ethics you can't simply say what they print is automatically rubbish.
If they say they have a source In M's inner circle then the reality is they do.
Whether the information provided by that source is 100% accurate is another matter, but the paper (having checked out that sources credentials) must have good reason to suppose that it is because otherwise its liable - in this case with a person who has already entered into legal action against them.