Well it was your comment that it was ”the same MI5 that gave us WMD’s” etc that made me respond.
In fact MI5’s general level of intelligence information is quite good – not flawless it’s true – but on the whole pretty sound.
The SIS (MI6) on the other hand is still smarting from the WMD issue.
So if MI5 openly tell us that they are currently investigating a large number of terrorist plots, my instinct is that they should be believed and allowed to carry on protecting us as best they are able.