"Jokingly, I replied it would depend on whether his drivers had any good stories for us," recalls Wallis.
"He said 'I've got a good story, my pregnant wife has been thrown into prison for a crime she did not commit'."
The cabbie's wife was Seema Misra, a sub-postmistress who had been sentenced so 15 months in prison for stealing £75,000 from her employer. Wallis did a bit of digging around, and discovered that Computer Weekly magazine had already carried reports that all was not well with the Post Office's flagship Horizon computer system. That cheeky pitch for a bit of minicab work would lead Wallis on a 13-year quest to uncover what has been dubbed one of the greatest miscarriages of justice in British history.
When Seema's conviction was overturned on April 23, 2021 – at the same time as 38 other former post-office workers – it was also vindication for Wallis who had set up a crowd-funding campaign to support his coverage of the case.
Last month's ITV drama, Mr Bates Vs the Post Office, has finally pushed the scandal to the top of the news agenda, with Prime Minister Rishi Sunak now pledging to intervene. And Wallis, who acted as a consultant in the making of the show, is now telling his story in a national theatre tour, which will take in Dudley, Shrewsbury, Stourport and Lichfield. At each of the talks, he will be joined by a former sub-postmaster who will share their own personal experience.
Wallis admits that it is quite an unusual way for a journalist to bring his part in the story to the wider public.
"I was approached by a friend who was a theatre producer who saw that this drama was coming," he says.
"He said people would be so outraged by this drama that they would want to know more about this scandal. I was a little bit wary, but he knows about this stuff, so I had better just trust his judgement."
The same month as Seema Misra was being sentenced in what she had been told was an isolated case, former Shrewsbury sub-postmistress Rubbina Shaheen was also jailed for 12 months at the town's crown court. Her conviction was overturned on the same day as Seema's. Wallis remembers being introduced to her and her husband Mohamed by Alan Bates – the eponymous hero of the television drama – at Dobbie's garden centre off the A5 in 2014.
"You couldn't want to meet a more lovely couple," he says. At one point the couple were reduced to living in a van, using the toilets in the local branch of Tesco to wash, and Mrs Shaheen's health suffered over the years that followed.
"They are such nice people, and they have been put through the mill," he says. "How can any amount of compensation make up for what they have been through?"
Also overturned the same day was the conviction of Tracy Felstead from Telford, who was just 19 years old when she was jailed for six months in 2001, convicted of stealing £11,500. She was held in a secure psychiatric hospital after twice trying to take her own life.
"That poor woman has spent most of her life as a convicted criminal," says Wallis. "They accused her of stealing more than £11,000, but when they went through all her bank records there was no record of her ever having had the money. She's a lovely lady, and I just hope she's able to get on with the rest of her life now."
Carl Page, who kept a post office in Rugeley, also had his sentence quashed on April 23, 2021, having been sentenced to two years in jail at Stafford Crown Court in 2007 for the theft of £94,000. But he believes the interesting case could yet prove to be that of Neela Hussain, who was jailed for 21 months for the theft of £21,000 in 2011. Hussain, who managed the post office branch in West Bromwich's Farley Centre, had her appeal held the same day as Mrs Shaheen, Miss Felstead and Mr Page. But unlike the others, her conviction was upheld, and Wallis does not think that is the end of the matter..
"Her conviction was upheld because it depended not just on Horizon evidence, and therefore the conviction could not be quashed. But that doesn't take into account the way these prosecutions were brought. It will be interesting to see whether her conviction is quashed when the new legislation promised by the Prime Minister comes into force."
The legislation Wallis refers to was the pledge made by the Prime Minister in the wake of the television drama to overturn all convictions brought about because of shortfalls with the Horizon system. Wallis says this is essential simply because of the amount of time it will take to hear the 600 outstanding appeals on a case-by-case basis.
"If the Court of Appeal hears this cases at the rate it has been doing so, many of these postmasters will be dead by the time their cases come to court," he says.
While the news media had been reporting on the campaign for justice ever since Computer Weekly broke the story in 2009, there is no doubt that the public's interest in the case has grown exponentially since the television drama, starring Toby Jones as Alan Bates was screened.
After more than 13 years recording the individual stories behind a great miscarriage of justice, it not just a little frustrating that it has taken a television drama to capture the mood of the public?
"Not really," says Wallis. "The drama did something which you can't do with a documentary or in the print media, it gives the characters involved an emotional resonance of what's been happening to these people."
"I think the drama is what we should be doing in a functioning democracy."
*Post Office Scandal – the Inside Story will be at Lichfield Garrick Theatre on March 24, Theatre Severn, Shrewsbury, on April 14, The Civic, Stourport on April 15, and Dudley Town Hall on April 19.