MI5 were not involved in this case.
Bit of a waste of time even debating this point.
The more interesting question to ask is why did a former PJ who once had status and respect say this?
My guess...
The McCann case humiliated him. His own did him in. The prosecutors thought he was laughable. More credible, talented colleagues in the PJ knew he was embarrassing the organisation. He was essentially shown up to be what he is - a small town cop who doesn't have the ability to run and coordinate a case of this magnitude.
Once he lost his status, he traded in his former life for speculation, fantasy, money making. He's trapped in an echo chamber of people telling him he is still distinguished and right. He's so right about the McCann case they tell him. Your documentary exposed the truth... Sad, because he craves the validation of the PJ who don't give him that... They know he's a joke, This rejection spurs him on to keep acting in such a strange manner. He knows the PJ can't comment and as such, the media will run with his stories because there is no other news about the investigation.
Sad really as i bet he did some good work in his day.