How on earth did Labour select Mandy Richards, a woman who spread conspiracy theories about the assassination of Jo Cox and the Manchester terrorist attack, as its prospective parliamentary candidate in Worcester, a key marginal seat?

That’s the question being asked of the party after Richards’ tweets – and the fact that she is subject to no fewer than 14 civil restraint orders – were published in the Sunday Times.

One important thing to note: in both the original Sunday Times story and elsewhere, Richards has been described as a Corbynite or Corbynista, with the Sunday Times describing her as a “strong supporter of Jeremy Corbyn”. That is not – from my knowledge of Richards’ own politics and indeed the general politics of Hackney, where Richards lives – an accurate descriptor, and indeed it mystified several people in the Hackney party who I spoke to this morning that she was described as such.

The truth is that Richards has been a perennial candidate for parliamentary selection since well before Jeremy Corbyn became leader and her politics have changed depending on the circumstances. Not that she is particularly lonely among ambitious Labour politicians seeking parliamentary seat in discovering a late passion for Corbyn: but it is merely to say that describing her as a “Corbynite” misdiagnoses the party’s problem.

Labour’s ruling national executive committee used to vet candidates before they went for selection, but this was abandoned after 2005, because of a widespread (and it must be said, largely justified) belief that as well as weeding out undesirables the process was being used by both central and regional offices to knacker the hopes of candidates who had blotted their copy book in some way shape or form.

That change meant that the task of vetting candidates fell entirely to the local parties. They have a brief to refer candidates they find to be alarming or unsuitable back up to region, but this doesn’t always happen.

Adding to that problem, Richards’ candidacy looked less worrying than it should have to the local party in Worcester because she had already been a longlisted candidate in Hornsey and Wood Green in 2013, where Catherine West was ultimately selected.

Why? In order to exert a measure of control of the process, Labour regional and head offices took a leaf out of the Conservative playbook for managing selections: presenting a shortlist of candidates in which the preferred candidate rubbed shoulders with a series of people who were considered highly unlikely to pose a challenge: a “managed choice” in which the roster was essentially: the unfit, the unsound and the preferred candidate. It was widely believed at the time that Richards’ participation in the Hornsey selection was an example of this. But I’m told that as far as the local party in Worcester was concerned, they regarded her participation in that contest meant that she was a clean candidate.

Adding to the problem, local parties made up of local volunteers and the long use of “vetting” as a cover for knobbling candidates can make it hard for local party officials to exert their powers, who feel under pressure to wave people through. A longstanding Labour rule that no question at hustings may be asked of specific candidates, only of all the candidates. There are good reasons for this rule but it of course means that even if one local activist discovers something bad about a candidate, they can't easily bring it up.

So the selection of Richards (likely to be shortlived, as the NEC is expected to refuse to adopt her as a candidate) is the result of several long-running problems: an abandonment of vetting by the centre and a distrust of “vetting” among the party grassroots, a willingness to use unsuitable candidates to ease selections elsewhere that then conveys respectability on them.