“This whole thing is a load of cobblers!” huffed a flustered Boris Johnson, sitting at his round table in London’s City Hall on Thursday afternoon. “We need to get on and build this bridge … you just cannot bear the idea that a great project is going ahead. That is what this is all about. A load of cobblers.”

It was the surreal climax to what felt like something of a festive garden bridge pantomime. The fairy lights of a Christmas market twinkled through the windows while tourists shuffled along the south bank clutching cups of mulled wine, oblivious to the heated debate going on inside the chamber. The mayor had been summoned, along with the head of Transport for London Mike Brown and its planning director Richard de Cani, to explain themselves to the Greater London Authority’s Oversight Committee over the murky genesis of the proposed garden bridge. And Boris, chief bridge booster, was getting increasingly hot under the collar.

“The whole tenor of this conversation is completely out of whack!” he barked. “You’re trying to cast a cloud … you’re just a bunch of—”

The cross-party committee was here to get to the bottom of why the garden bridge project has followed such an unconventional, fast-tracked process since Joanna Lumley first wrote to Johnson in 2012 with an idea for a fairytale crossing. Following a calm, level-headed line of questioning, they asked how the mayor first heard of the bridge idea; if he knew Lumley was an Associate at Heatherwick Studio [the bridge’s architect] when they met; how many meetings they’d had before the official tender was launched; why TfL’s own legal advice about how to procure the project was ignored; and why the strong criticisms of the process in the internal audit were removed.

It was a forensic grilling, aided by a weighty stack of confidential documents released after countless Freedom of Information requests, that was batted back by a combination of Boris bluster and procedural admin-speak from Brown and de Cani.

The chief contention hangs over how Heatherwick was selected for the project, and the suggestion that his scheme was a done deal from the start – which was a concern of TfL’s own legal team. “We are aware that Thomas Heatherwick has already raised with the GLA and TfL the possibility of a garden bridge,” TfL’s lawyers wrote in January 2013, in a letter giving advice for procuring “specialist design services” for the project. “Care will be needed to ensure that, particularly in relation to timing of the competition and its specification, other bidders have an equal opportunity to pull together all the relevant disciplines and to put their ideas forward.” The letter goes on to add that “it will be important to ensure that is not, and does not appear to be, the case that decisions have already been made about the proposed structures.”

The advice is crystal clear: “The procurement of the design team for the bridge will need to be subject to competition through OJEU [the EU public sector procurement mechanism].”

But that didn’t happen. Instead, TfL decided to split the process in two. In February 2013, it ran a small tender process to procure a design advisor to “help develop the concept” (which was capped at £60,000 to avoid going through the full OJEU process). Heatherwick was selected, receiving a higher score for bridge design experience than two other experienced bridge architects. His submission, which was also released as part of a FOI request, is not a feasibility study at all, but merely an elaboration of his own garden bridge design, with Lumley listed as an Associate of the studio.

Then in April 2013 a second tender was issued “to develop the technical design of the bridge, to enable a planning application to be submitted”. With a contract value of £8.4m, the process used the official TfL Engineering & Project Management Framework and went through the proper formal stages of Expression of Interest and Invitation to Tender. Engineering giant Arup was duly appointed – then subcontracted Heatherwick to its team. The contract with TfL ended in April 2015 and the project was transferred to the Garden Bridge Trust to deliver.

When pressed over why this unconventional route was chosen, as opposed to the one advised by TfL’s own lawyers, de Cani said: “The circumstances had changed. The advice was clearly not relevant because we were doing things differently.” He assured the committee that they took additional legal advice over the new process.

The original advice, he said, was given on the basis that TfL might run the project from start to finish, though that later changed. “That wasn’t the approach we adopted because there was the opportunity to get the private sector to pay for most of the project,” he said, “and that’s where the trust came forward.”

Perhaps this all sounds fair enough – unconventional and not following due process, but a pragmatic response to changing circumstances. Except that his explanation doesn’t quite tally with the timeline.

The Garden Bridge Trust was founded in October 2013: it simply didn’t exist when the tender process changed. The idea for a trust was first mentioned to the TfL board in July 2013, in the Commissioner’s report on the project. The report also specifically states that TfL would be responsible for the development and “enabling costs associated with securing the necessary consents and approvals” – ie it would run the project to planning permission stage, which is exactly what the £8.4m Arup contract represents. There was no suggestion that this stage of work would ever be handled by a private trust. And even if TfL had envisaged that the project might be handed over to a trust following planning permission being secured, this would in no way represent a “change in circumstances” around the procurement, as de Cani claims.

It might sound like a minor point of procedural detail, but it adds increasing weight to the suggestion that the entire process was rigged to allow Heatherwick’s proposal to prevail – an allegation that TfL continues to deny. Meanwhile, other questions were skimmed over with theatrical prevarications and hazy recollections. Master of misdirection, Johnson kept leading the conversation off-topic, trying to recall whether the delightful model Lumley had first shown him was made of glass or wood. Either way, it was very beautiful. And the lovely Joanna is a hugely public spirited person, who has been doing all of this for free, and doesn’t deserve to have her name dragged through the mud. And he couldn’t quite remember if she or Heatherwick had accompanied him on a 24-hour trip to San Francisco in January 2013 to try and secure sponsorship for the bridge – a trip which cost the taxpayer more than £10,000.

“You don’t start lobbying for investment in a specific scheme when it has not been selected unless you are absolutely certain of the outcome,” said committee member Caroline Pidgeon after the hearing. “When the mayor made this lengthy and expensive trip there were no official plans by TfL for a garden bridge. The tender document for an initial design of a pedestrian footbridge had not even been published. It is mockery to claim that it was an open and fair competition when the Chair of Transport for London was already flying around the world seeking investment for one proposal.”

In the huffing, puffing, insult-throwing court of Boris, the real reason for the garden bridge (and the speed at which it has been bulldozed through the system) became clearer than ever. TfL has always framed the project as a crucial transport link, not a private tourist attraction, nor a mayoral vanity project. But, bathed in a festive purple glow beneath City Hall’s chamber, Boris finally delivered his revealing pantomime soliloquy. “A great city like London has to continually refresh its offer,” he said. “It’s important that we don’t rest on our laurels, but continue to adorn the city with things that will attract visitors … and to get it done within a four-year mayoralty is a very challenging thing.”