“Traingate” seems to have developed as much from Guardian flaws as Guardian strengths.

The video of Jeremy Corbyn sitting on the floor of a train, disclosed exclusively online on 16 August, was mistakenly treated by the Guardian more as freelance journalism than what it actually was: a kind of gonzo news release by two Corbyn supporters.

The pair did not deal deceitfully with the Guardian. In their terms, the campaigners initially succeeded handsomely in promoting Jeremy Corbyn and his argument for transport policy change.

The Guardian was told at the outset that Corbyn had obtained a seat during his journey, but the information was not included in the news report which accompanied the video. Well after it had been established that Virgin Trains staff had organised a seat for Corbyn about 45 minutes into the journey, the original Guardian report still had the erroneous headline “Corbyn joins seatless commuters on floor for three-hour train journey”.

Although the Guardian did not intend to mislead readers, that was the effect for some time. Its pre-publication checks and balances failed in some respects. Post-publication, it was not quick enough to fix what it could, and to explain.

Had the Guardian done so, the episode may not have generated the controversy which acquired that exhausted suffix, -gate.

Had the news report not carried, unexplained, the pseudonymous byline of one of the campaigners, readers may not have felt the need to express concerns to me after reading in Private Eye of his past activism on Corbyn’s behalf.

These are my main conclusions after investigating how the initial story was handled. (I do not aim to settle the arguments about how crowded and pre-booked a train must be before it can fairly be called “ram-packed”.)

Let’s start at the beginning.

The freelance film-maker Yannis Mendez is highly regarded by Guardian staff who have worked with him. On 11 August he had a different client. He was being paid by the Jeremy for Labour campaign to make a video record of its work. In that role Mendez was part of the Corbyn entourage on the 11am Virgin Trains East Coast service from London to Newcastle, where Corbyn was to debate challenger Owen Smith.

After walking through carriages without finding sufficient seats together, Corbyn sat down on the floor. Mendez described this to me later as “a completely impromptu moment”.

Mendez says he filmed while Corbyn delivered straight to camera his message that: many travellers faced this problem daily; the train was “ram-packed”; staff were working really hard; more trains were needed; fares were expensive; it was a good argument for public ownership.

It lasted less than 30 seconds. I have seen the version Mendez supplied to the Guardian and it is the same as the published version except for the addition by Guardian staff of subtitles to help readers discern Corbyn’s words from the background sound of the moving train.

Mendez tells me that it was he who decided to push the footage into the media. He says he told the Corbyn campaign of his intention. According to its co-director, Sam Tarry, the campaign was not given an opportunity to sign off on the package of video, still images and text which was later offered to the Guardian.

The video was prepared by Mendez and still images and text were compiled by his friend Anthony Casey, who writes under the name Charles B Anthony and is a passionate Corbyn supporter. Mendez calls him Bobby. Here, we’ll call him Anthony.

Anthony was not on the train. Mendez says he phoned Anthony during the journey and Anthony advised him to gather contact details for people also on the train. Anthony later obtained social media posts, two by women who had been photographed on the floor next to Corbyn. From Mendez he received still images of a young couple also sitting on the floor. He gathered screenshots of three tweets dating from 2015: two showed images of crowded trains and one combined an image of George Osborne in a train seat with one of Corbyn standing on a night bus.

Around the video and these images Anthony wove almost 500 words in praise of Corbyn, together with quotations from Corbyn. The style is sufficiently apparent from these excerpts:

“Overcrowded trains, we’ve all faced them. Crushed up against our fellow commuters, standing in the aisle with a person’s armpit in your face, sitting in the luggage compartment or pitched up next to an overflowing bin. It’s an everyday occurrence for many, but you don’t normally expect to see the leader of the opposition sitting on the floor of a carriage on your way to work.

“… Halfway through the journey the train emptied and Corbyn and his team managed to grab some seats, but before Corbyn could rest comfortably and prepare for his duel with Smith he offered a seat at his table to the gentleman next to him, also without a seat.

“… Jez, keeping it real.”

Anthony tells me he prepared the material this way because he had decided to pitch it to Buzzfeed first. When Buzzfeed declined, Mendez contacted the Guardian and offered exclusive use of the video on condition that Charles B Anthony’s article also be accepted.

Mendez did not seek payment for himself; he was being paid by the Corbyn campaign. He did raise the issue of a standard freelance payment for Anthony. I’m told by the relevant Guardian staff that no invoice has been received and no payment made. Anthony says he did not seek payment himself and is happy enough to have his byline in the Guardian. Tarry confirmed that the campaign paid Mendez but not Anthony.

The Guardian did not think carefully enough about the status of the material being offered, nor the condition attached. By the time the material reached the national news desk it seems to have been assumed that Anthony would get a byline. The acting editor that day, Paul Johnson, was not consulted. Coming from a trusted freelance (Mendez) and accompanied by images and words from a person known to that freelance (Anthony), the material seems to have been viewed more as unpolished journalism than untreated promotion.

The result was that the Guardian applied less scepticism and fewer checks than should normally be applied before a news story of this sort is published.

Guardian editorial standards necessarily treat differently:

• freelances, who when contracted are bound to the same standards as staff;

• sources, whose motives always need careful weighing, especially if they are to be anonymous;

• pseudonyms – to be used only exceptionally, with the permission of senior editors, and disclosed to readers;

• material known as “user-generated content”, often videos made by members of the public at newsworthy events and posted to social media or provided directly to mainstream media outlets.

The Guardian’s dealings with Mendez and Anthony can be fitted into several of these categories, but that feels strained. My conclusion is simpler: the pair were acting as unorthodox public relations operatives for a prominent, campaigning politician and their material should have been treated like that.

Staff who worked on the story say they did not know that “Charles B Anthony” was a pseudonym or that Buzzfeed had declined the material. Johnson tells me he is concerned the Guardian learns the right lessons from this episode. “We should always follow our editorial guidelines when considering the use of byline pseudonyms – if we have prior knowledge. Transparency for the reader is vitally important.”

As a freelance, Anthony was an unknown quantity. The desk editor that day, who received Anthony’s copy from Mendez at 11.51am, assigned a staff reporter to work on the material. Anthony and Mendez were contacted. One of the women who had sat with Corbyn and posted about it was interviewed by phone.

When the reporter learned that Anthony had not been on the train, doubt was raised with the newsdesk about relying on Anthony’s information. Participants’ recollections differ about how much concern was expressed. Johnson was not consulted. He reaffirmed to me later what the editorial guidelines also emphasise: staff should feel they can express any misgivings about a story.

I think the rush-to-publish, which is at once so wonderful and so perilous in journalism, ought to have been slowed. The material was exclusive, so the natural urge to beat the competition was not the factor it may otherwise have been.

The desk editor told the reporter to include in the story a disclosure about Mendez working for Corbyn. This was necessary and sensible, as far as it went. But the article was silent about the Charles B Anthony who shared the byline. Anthony’s raw material left no doubt he was a partisan for Corbyn. Readers ought to have been alerted to that fact. About 100 of his words made it into the rewritten article of about 360 words, and although Anthony’s strong promotional tone was reduced, more should have been done for this, a news story.

I have followed the Anthony text from its raw state, through the staff reporter’s rewrite, on into the editing stages and finally to its published form (online at 2.59pm on 16 August and on page 4 of the paper on 17 August). The information about Corbyn getting a seat was left out during the staff reporter’s rewrite. The reporter tells me the reasoning was that neither the reporter nor Anthony had been on the train, the aim was to report accurately with the information the reporter had been given, and the copy was focused as much as possible on what could be seen and heard on the video.

Even allowing for time pressure, this was a misjudgment. Anthony’s sentence about the Corbyn team getting seats was inaccurate in its reason, about the train emptying halfway, but that sentence put the Guardian on notice that Corbyn did get a seat. More should have been done to verify how and when it happened. Neither Corbyn nor Virgin Trains was contacted pre-publication to check.

The desk editor tells me he did not notice the omission.

So the online headline was wrong from the outset. It should have been frankly corrected straight after the significance of the omission was realised, which seems not to have occurred until a week later.

On 23 August Virgin Trains challenged Corbyn by releasing CCTV footage from the train. The Guardian’s original story was widely and prominently debated across mainstream and social media. Anthony tweeted that day that his original copy for the Guardian had included the fact that Corbyn had managed to get a seat.

Next day, online and in its front-page account in the paper, the Guardian reported one of the women who had sat on the floor and been photographed with Corbyn as saying that a seat had become available for him 45 minutes into the trip when staff shuffled people around. That straightened the record, but not as transparently as the Guardian’s role in the controversy required. It was a significant error and should have been expressly corrected as soon as possible, as the editorial guidelines state. Following this review, it has been corrected.