Later in the meeting TfL denied that it had put words in Sir Terry’s mouth.

Sir Terry claimed he agreed to take the TfL line at the time so that he could tell Mr Khan, who was not present at the board meeting, personally the next day.

What was said at this meeting on – 26 July – which was also attended by London’s deputy mayor for Transport, Heidi Alexander, continues to be the focus of the greatest controversy.

Despite publication of the presentation given primarily by Crossrail’s then chief executive, Simon Wright, which shows that even a partial opening was ‘not feasible’, Mr Khan and Ms Alexander appear to have staked their defence on a claim there were contridictions in what they were told.

However, on a page headed ‘Options for opening in December’ the presentation given by Mr Wright and Sir Terry states ‘2 Options for partial or sectional opening in December have been analysed… They are judged as not feasible’. No other options for opening in December are cited as having been considered.

The defence

Ms Alexander claimed what was said verbally contradicted the idea that a December opening should be ruled out, suggesting instead that Crossrail was still seeking to achieve a solution.

At a Transport Committee meeting last month, Mr Khan claimed that Sir Terry had ‘misremembered’ what was said at this meeting.

Sir Terry told assembly members that he was disappointed by the personalisation of the dispute, adding: ‘I do not suffer from loss of memory.’

Ms Alexander appeared before the committee after Sir Terry and was questioned by Conservative assembly member and 2020 mayoral candidate Shaun Bailey.

Ms Alexander did not deny directly that Sir Terry had told her and Mr Khan at this meeting that the December opening date would not be met.

Given that the onus would appear to be on herself and the mayor to prove that something was said that disproved what the documents suggested, she started with a worrying degree of uncertainty: ‘I think my recollection of what happened at that meeting….’

Eventually Ms Alexander said that Sir Terry ‘certainly hadn’t taken a decision to rule out a December opening, because if he had, as was explained at the session that took place on the 21st of December, he would have had to have issued an adverse event notice'.

'He said that he had instructed his executive to go away and do more work – you know you can’t make an announcement if you’ve got some half-baked half story and that was the nature of the discussion that took place in that meeting at the end of July.’

Undoubtedly Ms Alexander’s reference to ‘a decision’ to rule out a December opening was carefully chosen as a non-denial, to allow her to exclude such a formal conclusion on the grounds that it was not followed by what would have followed it. But Sir Terry had not claimed to have made a formal decision, just to have laid the situation out clearly.

In fact, he had previously explained that Crossrail Ltd had not issued an adverse event notice after its July board meeting because it did not at that time have full costings for the delay. Ms Alexander’s comment that ‘you can’t make an announcement if you’ve got some half-baked half story’ backed this up, seemingly without her realising, and significantly undermined her reliance on the absence of a formal ‘decision’.

The new CEO

Mark Wild, who has since become Crossrail’s CEO, was at the time a TfL nominee on the Crossrail Board and responsible for communicating what was going on to TfL and City Hall.

He told the committee he had thought Crossrail executives, who had overcome significant issues in the past, might have been able to do something: ‘Is there a possibility of opening a small bit of the railway? Is there a possibility of omitting some stations? Could you take some of that complex functionality out?’

This appeared – and was no doubt intended – to support Mr Khan and Ms Alexander in their claim that they were told that a December opening was still possible.

Mr Wild was keen to stress that no-one involved was seeking to conceal anything and that the main issue was not one of communication but the delivery of the project.

He added that last summer no-one involved, including Crossrail executives, realised how very far from completion the project was - well beyond the issues that were initially said to be behind the delay.

He said: ‘If I’d known then what I know now, I’d have set the fire alarm off.’

Ms Alexander had a rather less forgiving view of this, arguing that ‘the real story of the last six months, the real story of Sir Terry’s chairmanship of Crossrail’ is how the project went from an assurance given to her last spring that ‘not a penny more’ than the additional £211m agreed at that time would be needed to complete the project to the situation in November of needing a further £2bn financing – ‘that’s a multiple of ten’.

Logic suggests that one of the following versions of what happened, is true which look equally worrying:

a) Everyone knew and kept it hidden for too long

b) Sir Terry knew and kept it hidden from the mayor's office

c) Sir Terry told the mayor's office, which is now trying to pretend it didn't know in order to shift the blame

d) Sir Terry knew and tried to communicate it but the message was obstructed by officials 

e) There was a straightforward communication breakdown, despite people's best intentions

f)  No one knew and the project went off track without anyone fully realising