Sitting outside a pavement cafe, Mark Daly is approached by a passer-by.

"I just want to congratulate you on your journalism," the man says, extending his arm to shake Daly's hand. "First-class work. But can I ask a question? Why did Ticketus give that money to Craig Whyte before he owned Rangers?" Squinting in the sunlight, Daly smiles and answers the question as best he can.

The story of Rangers' descent into crisis and the subsequent struggles to survive has become an epic saga. The cast of characters has been diverse, from Sir David Murray, a dominant figure in Scottish business and public life, to Charles Green, a gruff Yorkshireman who was hounded out of Sheffield United but was now on the verge of buying the Ibrox club. Since Duff & Phelps were appointed as Rangers' administrators last February, Daly has been investigating the story, and the evidence he uncovered was aired in BBC Scotland's documentary, The Men Who Sold The Jerseys, last Wednesday night.

It alleged David Grier, a senior partner at the accountancy firm MCR advising Craig Whyte on his takeover last year, knew that money borrowed from the London firm Ticketus was being used to fund the purchase of the club. His colleagues at MCR were Paul Clark, David Whitehouse and Sarah Bell, and all four were retained when the company was bought over by Duff & Phelps last November. Grier, however, claims he did not know money being borrowed from Ticketus was being used for anything other than working capital, even although Daly claims emails he uncovered suggest otherwise.

"It turns out that some people were quite keen for stories to get out," Daly says. "It's a cliche in journalism when you talk about brown envelopes arriving at your door, but that was what was happening. There were also people whom I kept anonymous and whom I grew to trust who were helping me, giving me information and documents.

"No journalist, especially not at the BBC, can or should be allowed to make unfounded allegations. Every single line in the script of the film was tested, was signed off by various levels of editorial management and most importantly the lawyers. If there were things about the way Duff & Phelps have operated in the past that could affect their ability to do their job neutrally, people had to know about that."

After Grier and Clark denied Daly's allegations, the BBC released more of the information uncovered during the investigation, including further emails and details of a celebratory lunch at the London restaurant, Langans, attended by nine people. The BBC claims that eight of them – which included Whyte, Phil Betts and Gary Withey, who became Rangers directors, and Ross Bryan of Ticketus – knew how the takeover had been funded. The ninth attendee was Grier.

The Insolvency Practitioners Association is launching its own investigation into Duff and Phelps. Grier and Clark now say they are taking advice from their solicitors on the programme, while they also claim they offered Daly evidence that would disprove the allegations but, smiling, he refutes this.

"Many times I wrote to their PR guys begging them to show me the evidence they had that would show me I've got it wrong. They told me that for legal reasons they couldn't provide them and were only happy to do it in an off-the-record briefing. That's not good enough. Come along, be interviewed, show me the documents in an interview. We don't make allegations in the hope people don't sue us, we make allegations knowing we can back them up."