>> the report lays the blame for that squarely on MI.5.
> No, they don't. It's not up to MI5 to investigate this sort of thing without being asked.
Paragraph 34 in the report:
> In our opinion, the operational role must sit primarily with MI5, in line with its statutory responsibility for “the protection of national security and, in particular, its protection against threats from espionage, terrorism and sabotage, from the activities of agents of foreign powers and from actions intended to overthrow or undermine parliamentary democracy ... ”.
The issue seems to be that MI5 had a lot on its plate. The report talks about changes in percentages of time spent looking at Russia over the years. With percentages, it's difficult to tell actual numbers - they may have spent more actual time if they had a big increase in resources - but the assumption is that with the rise of international terrorism, MI5 spent a lot more time looking at that and a lot less time looking at Russia. They took their eye off that particular ball, as others were lobbed at them.