With protesters gathering and media cameras carefully angled, one of the most important people in the whole Post Office Horizon IT scandal will sit for three full days of questions.

Wednesday is the start of the moment sub-postmaster victims, and likely anyone involved through the years the Post Office injustice was perpetrated, have been waiting for. It’s been five years since the Post Office apologised but victims are awaiting redress and answers they hope Paula Vennells may provide.

Why does Paula Vennells matter?

Former chief executive Ms Vennells was at the helm of the government-owned body during the key Horizon operating years of 2012 to 2019.

She’s been regularly referenced in the inquiry set up to establish a clear account of the introduction and failure of Fujitsu’s Horizon accounting software.

Horizon wrongly generated shortfalls at Post Office branches and led to hundreds of false accounting and theft prosecutions. Many more sub-postmasters racked up significant debts, lost homes and livelihoods, became unwell, left communities and some took their lives as they struggled to repay imaginary losses.

While this is the first opportunity for inquiry barristers to publicly question Ms Vennells, hers has been a continuous presence through the documents presented to dozens of witnesses and the answers they provided.

A previously unknown name, Ms Vennells may now be familiar to the millions who saw a dramatised version of her portrayed in the ITV drama Mr Bates v the Post Office that revived interest in the injustice.

In the wake of the show Ms Vennells, an ordained vicar, gave up her CBE (Commander of the British Empire) and reiterated her apology and regret for the harm caused to sub-postmaster victims.

As she agreed at a government select committee in 2015, the buck stopped with her.

Did she turn a blind eye or take part in a cover-up?

The issue of what Ms Vennells knew and when has been the subject of news reports which detailed the extent of her knowledge of the scandal, years before prosecutions were halted and an apology was issued.

Whether Ms Vennells sought to suppress or minimise evidence or just overlooked it will shed light on why the scandal ran on for as long as it did – from when sub-postmaster and advocate Alan Bates raised issues in 2003 up until 2019 when an apology was issued.

When did she first know sub-postmaster accounts could be altered remotely?

Key to understanding why Ms Vennells acted as she did is when exactly she knew the Post Office’s IT helpdesk or people in Fujitsu could access and edit Post Office branch accounts.

Why did she allow prosecutions to go ahead on the basis there was no remote access, despite legal advice?

Whatever her answer, there’s evidence – in the form of recordings leaked to Sky News – to suggest Ms Vennells had been told of remote access by May 2013, at the latest.

But three years earlier, in 2010 and before Ms Vennells’ tenure as CEO, Post Office prosecutors were alerted to bugs with Horizon, just days before the trial and eventual conviction of sub-postmaster Seema Misra, who was pregnant at the time.

Issues around Post Office convictions were again raised during Ms Vennells term when Simon Clarke, a barrister for a firm advising the organisation, wrote in 2013 that an important Fujitsu witness failed to disclose he knew of bugs, “in plain breach of his duty as an expert witness”.

This put the Post Office “in plain breach of its duty as a prosecutor”, he told the company in his formal legal advice.

Did she authorise £300,000 of legal spending to go after a £25,000 loss?

Sub-postmaster Lee Castleton, recognisable from the Mr Bates Vs The Post Office drama, will be particularly keen to know if Ms Vennells – as former managing director Alan Cook told the inquiry – signed off legal costs of £300,000 to prosecute Mr Castleton for a supposed £25,000 shortfall when she was a network director at the Post Office.

What’s her account of how she got it so wrong? Why did she allow the scandal to continue?

Given the evidence to suggest Ms Vennells was aware of bugs and defects in Horizon years before prosecutions stopped and an apology was made, members of the public and victims alike will want to hear her account of why she did not act to scrap Horizon.

Why did she not act, and apologise, earlier?

Many will want to know why she had such faith in Horizon, Fujitsu and those working for the Post Office when sub-postmasters, MPs representing constituents, legal advisors, and even Second Sight, the forensic accountants hired to investigate were telling her there were problems.

What did she think of sub-postmaster complaints against Fujitsu?

Ms Vennells was clearly not so concerned about Horizon that she did anything to minimise its role, not least end it. So what did she think of what sub-postmasters were telling the organisation they were going through – did she think they lacked credibility, or perhaps that they were small in number and easy to ignore?

Why was she closed to the idea of faults in Horizon?

Horizon shortfalls had been discussed at the Post Office for years – why did Ms Vennells believe it was to be trusted over hundreds of sub-postmasters? How did she come to conclude Horizon was robust and claims against it were not?

Why did she say in 2020 the Post Office ‘did not identify’ defects with Horizon?

We do have an understanding of how Ms Vennells viewed the role of the Post Office and its oversight of the scandal – it’s one of ignorance. Since she stood down in 2019 Ms Vennells said the Post Office was unaware and that’s one of the things she’s apologised for.

“I am sorry for the hurt caused to sub-postmasters and colleagues and to their families and I am sorry for the fact that during my tenure as CEO, despite genuinely working hard to resolve the difficulties, Post Office did not identify and address the defects in the Horizon technology,” she wrote in June 2020.

Why did she say this when there’s evidence the Post Office did know?

Follow the questioning of Paula Vennells at the inquiry live on Sky News on Wednesday. Watch Sky News live here, and on YouTube, or on TV on Freeview 233, Sky 501, Virgin 603, and BT 313. You can also follow the latest on the Sky News website and app.

Why did she tell Parliament there was ‘no evidence’ of ‘miscarriages of justice’?

There’s a lot to be asked about Ms Vennells previous statements. Top of the list for many will be her answers to a February 2015 meeting of what was then the Department for Business, Innovation and Skills (BIS) select committee.

At that point – after forensic accountants Second Sight had uncovered and informed her of Horizon bugs – she told the MP committee members there was “no evidence” of “miscarriages of justice”.

Why were forensic accountants, who were getting to the bottom of Horizon issues, sacked?

Sub-postmaster advocate and former MP Lord Arbuthnot said he believed it was because they were getting too close to the truth.

Why, when she said she was going to ‘focus fully on working with the ongoing government inquiry’, were her lawyers giving documents to it hours before hearing evidence?

When an inquiry was announced into the scandal in 2020, Ms Vennells said she was going to “focus fully on working with the ongoing government inquiry”.

The inquiry had set a deadline by which all relevant documents were to be submitted, however, 50 additional documents were submitted on behalf of Ms Vennells at 11:17 pm on Thursday night and continued to come on Friday.

Outstanding questions from an earlier inquiry

Another grilling of Ms Vennells was due to take place in March 2020 by MP members of (what was at the time called) the Business, Energy and Industrial Strategy (BEIS) Committee.

Given the evolving COVID-19 virus crisis, the hearing was postponed but questions were still asked of Ms Vennells by letter rather than in person.

A number of those questions were not answered.

Committee chair Darren Jones had asked 17 questions but only received 13 answers in her June 2020 written reply.

Whereas she responded to his other questions, these ones received no reply:

• How would you answer those sub-postmasters and postal workers who said that the Post Office investigation branch was more interested in asset recovery than finding the source of errors in Horizon and that they felt they were treated as if they were guilty until proven innocent?

• Did the Post Office Ltd board review the approach and attitude of Post Office investigators at any point during your tenure as CEO? If so, how many times and what was the outcome?

• Were you comfortable as Post Office Ltd CEO that your organisation was prosecuting sub-postmasters without recourse to the CPS [Crown Prosecution Service]?

• The judge in Bates v Post Office stated that Post Office Ltd had operated with a culture of “secrecy and excessive confidentiality”. Did you as Post Office Ltd CEO oversee a culture of “secrecy and excessive confidentiality”; Was Post Office Ltd, as the judge stated, fearful of what it might find if it looked too closely at Horizon?

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“I continue to support and focus on co-operating with the inquiry,” a statement from Ms Vennells said.

“I am truly sorry for the devastation caused to the sub-postmasters and their families, whose lives were torn apart by being wrongly accused and wrongly prosecuted as a result of the Horizon system.”

“I now intend to continue to focus on assisting the Inquiry and will not make any further public comment until it has concluded,” she added.