Since 7/7 there have been three successful terrorist attacks which, to me suggests that the security services have been pretty successful. There's no such thing as perfect protection from terrorism - suicide bombers in particular - you have to accept that occasionally you will fail. Then you must learn from your mistakes and start again. What you shouldn't do is go round claiming our security services are not fit for purpose. That kind of panic is useless and even counter productive.
I have never said they aren't fit for purpose. But we have to ask questions.
This attack is different to most of the other terrorist attacks that 'slipped through the net', such as the Westminster bridge attack and the Jo Cox murder. In those cases the perpetrators were (as far as I'm aware) not known to the security services, were very much under the radar, and acted alone or in a pair using very unsophisticated or readily available weaponry. Under those circumstances it is understandably exceptionally difficult for the security services to prevent a determined attacker.
The Manchester bombing is entirely different - clearly an established network, with the key perpetrator known to the security services and on their 'watch list'. And hardly under the radar - he seems to have almost been waving a flag saying 'I'm a potential threat' - indeed there are reports that he literally did that - wave a flag outside his house in support of Islamist extremists.
So I didn't have concerns about failures to detect and prevent in the Westminster bridge attack and the Jo Cox terrorist attacks. I do have concerns over this one.
I think it is perfectly legitimate to ask why the security services failed to prevent this attack, and to do so does not spread panic, but recognises that sometimes challenging questions have to be asked and unless they are we won't be able to keep people in Britain as safe as they could be.