Labour politicians, most prominently Yvette Cooper, complain that the Coalition Government junked Labour’s Imprisonment for Public Protection Sentences (IPPs), thereby making it harder for such dangerous terror suspects as Usman Khan to be held indefinitely.

That is true enough as far as it goes, but is a very long way from being the whole story.

Conservative politicians, including Priti Patel, counter that Labour, in government, brought in automatic release for sentenced terrorists, which the Coalition ended – but that Khan gained his release because of Labour’s policy, under the terms of which he was released: a fact which Cooper deliberately side-steps.

Again, this is correct – but it misses the main point.

Which is that, as David Davis pointed out on this site yesterday, judges Leveson, Mitting and Sweeney blundered when they ruled in favour of Khan’s appeal against the IPP which was containing him.  As he writes, if those judges had not decided as they did “the likelihood is that Khan’s two murdered victims would be alive today”.

At any rate, Khan gained, once the judges had made their decision, from a fateful gap in the law.

Had the Coalition’s end to automatic release applied retrospectively, it would have applied to Khan – and he might not have been free to murder and maim last Friday.

But it wasn’t.  It was applied only to new cases, not to old ones.  And since Khan’s was an old case, he was dealt with, once his appeal succeeded, under the law as it stood before the Coalition changed it…

…Which meant that Khan would automatically be released in due course (after eight years of a 16 year sentence).  Which he was.  Which left him free able to attend Friday’s conference.  At which he maimed and murdered.

Frankly, the Coalition messed up, albeit for understandable reasons.  Retrospection is a dangerous principle.  We understand why the Ministers of the time may have wanted to avoid it.

But it is sometimes necessary – for example, and as in Khan’s case, for national security purposes.  Some people are simply so dangerous that holding them indefinitely can be justified.

Obviously, there must be safeguards (perhaps an oversight regime involving judicial recommendations).  But the principle is well-established: consider the 18B regime during the Second World War.

This returns us to the bigger, wider argument that we made yesterday – that now is the time to deliver on the other half of Taking Back Control: in other words, dealing with ECHR abuses as well as EU membership.

The root of the problem in Khan’s case wasn’t whether he was or wasn’t or should or shouldn’t have been eligible for automatic release under a fixed term sentence.

Rather, is it that the law is subject to a tug-of-war between Ministers, who have a responsibility for keeping the country safe, and pro-Islamist lobby groups, who ultimately don’t care if men like Khan are a risk to innocents.

Human rights as intepreted under the terms of the ECHR mean that it is harder than it should be for the Khans of this world to be held indefinitely (with safeguards) rather than given fixed term sentences (even without remission).

Which is why elected MPs, not ECHR judges, should have the final say – whether the matter in question is votes for prisoners, say, or detention without trial.

Ultimately, the policy aim should be for a British Bill of Rights, which would be written under a Conservative majority government to strike a better balance, inter alia, between national security and civil liberties.

Boris Johnson published a useful Twitter thread yesterday which set out the background to the Khan case.  It was more impressive than the Government’s policy response.

To speak plainly, briefing that the Prime Minister intends to “lock up terrorists and throw away the key” is low grade.  He isn’t going to do it. And the judges wouldn’t let him even if he tried.

This is the politics of taking voters for fools – a symptom of why our entire political system is falling into public contempt.

Obviously, time served should more closely reflect sentences passed.  But that principle is beside the point in Khan’s case, and in those of others – since they should be behind bars until or unless they are no longer a threat.

Johnson can do better.  Indeed, he already is.  Page 48 of the Conservative Manifesto prepares the way for curbing the abuse of judicial review and getting the balance right between the ECHR and Parliament.

It is all there.  And since Johnson has prepared the way to travel, all he needs to do now, if he wins the election with a working majority next week, is to take it.