A common lament from British Labour MPs as they watch Jeremy Corbyn tighten his grip is that the cranks who lurked at the back of party meetings for 40 years, handing out angry leaflets from grubby carrier bags, have suddenly taken over.

If so there must be limited sympathy for the victims. Why did they keep letting cranks into their meetings year after year, even as the Blairite centrists tightened their grip?

Those leaflets the MPs mention were often published by rival parties, which alone should have had their bearers shown the door.

Calls for revolt against legitimate Labour leaders or policies need not have been humoured either. In a parliamentary system, ‘party democracy’ does not mean universal suffrage – Labour is as free as any members’ club to set its own rules, and the first clause of its constitution requires it to organise through Westminster.

Corbyn himself was Labour’s most rebellious MP during its last period in office, defying the whip 428 times in 13 years. That could easily have been grounds for withdrawing the whip. Now Corbyn is in charge, MPs who defy him face imminent deselection.

Why did Labour continue incubating the hard-left cuckoos in its nest, especially after the mid-1980s, when it briefly stood up to them after recognising their danger?

Complacency is an understandable factor – nobody saw Corbynism coming – but greed and cowardice are the original sins. The greed comes from assuming that in a two-party system, every voter left of centre is yours by default.

From this comes the related cowardice of not challenging the back of the room, because that might require telling awkward home truths to a wider audience.

Labour lost the last general election through an absurd inability to concede any limits on public spending, which in turn followed five years of pandering to anti-austerity placard wavers. Unlike Ulster, Labour never says ‘No’.

It is deliciously appropriate that Corbyn received congratulations from Gerry Adams and Martin McGuinness upon his re-election as leader last Saturday. No major party in the UK has shown Sinn Féin’s discipline and courage in rejecting its swivel-eyed fringe.

Of course, to most of Britain and Ireland, the Shinners are the people at the back of the room – with techniques of control to match. Within the republican party’s own frame of reference, however, it indulges no militant tendency.

Dissenters may be engaged with in the first instance but they are quickly frozen out or expelled if they do not snap into line. Notably, this has happened to prominent members and representatives as the leadership has shifted the movement’s centre.

Sinn Féin will also make its defence of the centre public – McGuinness’s 2009 denunciation of dissidents as “traitors to Ireland”, after the murder of a police officer, was a home truth that reverberated across the republican constituency.

On the day of Corbyn’s reelection, dissident republicans launched an all-Ireland party. Known as Saoradh, it is a socialist, non-electoral “political vehicle”, endorsed by the New IRA and clearly viewing Sinn Féin as the enemy.

If Sinn Féin was like Labour, this would all be going on at the edge of party meetings, organised by people who would still see Sinn Féin as the enemy.

Unionists think they have nothing to learn from this. The DUP and UUP both take almost perfectly overlapping broad church positions, greedily assuming they can appeal to every voter in one half of a divided society. From that comes the cowardice of never facing down the extremes.

Last Saturday – a busy political day – agreement was reached over the three-year parade stand-off in Ardoyne, Northern Ireland’s last major parading flashpoint.

Unionist politicians welcomed the news but could claim no credit for it. The deal was delivered because Sinn Féin stood up to dissidents and loyalist paramilitaries gave an ultimatum to the Orange Order.

Unionists do not give public ultimatums to loyalists or Orangemen, although many loyalists are Orangemen and many Orangemen are present at party meetings. There is scant evidence of private ultimatums either. On the contrary, Orange and fringe Christian elements still act as if they can expect great influence.

Unionists may condemn loyalist activity in general terms, usually with reference to ‘all sides’, but they struggle with even the mildest criticism of specifics.

Adams used to say unionism needed to “find its de Klerk”, a typically bumptious wind-up from Ireland’s faux Mandela.

But what if unionists found their Neil Kinnock – someone to replicate the former Labour leader’s electrifying 1985 conference speech, when he challenged the militants head on?

The DUP and UUP might ask what problem this would solve, given each party’s solidly centrist leadership.

Unionism’s problem is that the front of the room has humoured the back for so long that everyone in it looks like a crank.