If you allow me to explain the elements for a moment..
To prove Defamation:
A claimant needs to show that the statement complained of:
1. is defamatory, meaning that an ordinary person would think worse of the claimant as a result of the statement;
2. identifies or refers to them, and
3. is published to a third party.
1. The collation of existing commentary, compiled in to a report/essay with additional commentary, ticks the box of making a statement. There are decided cases for re-tweets for example and there is also the concept of defamation by innuendo or the "nod, nod, wink, wink rule" as it's sometimes referred. Anyone reading the 40 page report will conclude point 1. No matter what your opinion of him, it's nasty stuff and much of it is disputable. Just look at the division of opinion here on him.
2. Pretty easy - he is named.
3. Note - Point 3 does not say, made public, merely published to a 3rd party. If I tell you, you are a pedo for example, I do not defame you, if I tell 1 other person, its different. It only need to be created and distributed. The report was compiled and distributed to a supposedly independent risk board, who took decisions based on the content. Additionally, information was shared more broadly and once NF complained to the press, the contents used to brief execs. Lastly we have the false statement made to the BBC. NF then used the SAR to obtain the evidence, which he published to defend himself. That is point 3 ticked.
So defamation is established, you then look at the lawful authority and excuse elements. You are welcome to have a look, I've published links to the act several times. I doubt it will take you long to conclude that none have any merit.