tuc,
to be fair, I think the SI Report is one of the most thorough I've seen. Complete in terms of their remit, but not stepping beyond what they have powers to do. Without a lot more, probably unauthorized investigation, I can't see what further evidence they could present. They couldn't look at what you know, nor could they have used anything presented in other enquiries as evidence.
Whilst I fully understand your wider perspective on things, I'm afraid none of that was presentable evidence for them.
Also, not saying this has happened in this case, but the SI Report can sometimes be subject to change before it is agreed and published. My BOI Report was.