If it is true that News International is confronting the phone hacking scandal with a new zeal, we can only welcome it. The pity is that it has taken four and a half years to reach this point, not least because, as any detective or criminal lawyer will confirm, in that period useful evidence is likely to have been lost.

This makes it all the more important that, whatever other investigations may be taking place, there should also be a proper inquiry into how the company conducted itself between the arrest of its royal editor in August 2006 and the onset of this new zeal. A key concern must be the repeated assurances given by company executives that in this long interval they did everything in their power to track down any evidence that anyone on their staff other than Clive Goodman might have been involved in phone hacking.

Les Hinton, former chief executive of News International, gave such an assurance to the Commons select committee on the media in 2007. He repeated it to MPs in 2009. Colin Myler, the editor of the News of the World, gave similar assurances; so did Tom Crone, his legal affairs director, and so did Andy Coulson, editor at the time of Goodman’s arrest. You can read what they said in the oral and written evidence section of the committee’s report here.

That evidence raises a lot of questions. To take a simple example, a point made by Myler and repeated by Hinton was that the internal investigation involved examining 2,500 emails. The implication was that this showed thoroughness, but 2,500 emails in an organisation like the News of the World must be a drop in the ocean. One reporter might receive 2,500 emails in a matter of weeks. So why were only 2,500 emails examined back in 2006-7?

More generally, there is the role of the solicitors, Burton Copeland. Coulson said (Q1719) he brought them in very quickly to find out “what the hell had gone on” and gave them everything they needed. Crone pointed out to MPs (Q1388) that they were “probably the leading firm in this country for white collar fraud”. That sounds very vigorous and thorough. You might infer that this top firm went in and, with full management support, carried out a top-class internal investigation, leaving no stone unturned.

But police evidence to the MPs left a very different impression of Burton Copeland’s role. Detective Chief Superintendent Philip Williams, in a number of answers (see Q2010) portrayed the firm, not as independent investigators or as facilitators for the police investigation, but as lawyers acting in defence of the company’s interests, and he implied that they were a good deal less helpful than the police hoped.

A subsequent written submission (Nov 2009, Q6) by Assistant Commissioner John Yates put it bluntly:

News of the World instructed lawyers to respond to our requests for disclosure and they took a robust legal approach to our requests and provided material strictly based on the evidence against Goodman and Mulcaire.

So that’s another question. Were Burton Copeland there as independent crimebusters called in by the company to track down any possible traces of wrongdoing, as News International executives told MPs, or were they solicitors representing the corporate interest in unwelcome dealings with the police? Whatever it is that they were tasked to do, I’m sure the firm did it professionally and ethically, but we need to know.

This is not a matter of idle historical interest. For one thing, it is reported that Burton Copeland are acting for News International in this matter again today. More fundamentally, questions of this kind are crucial to the public understanding of News International’s conduct and they may have a bearing on the evidence that is available to police today.

So who is going to ask these questions? So far as I can tell they are not within the remit of the new CPS and Met investigations, and we are surely not relying on Rupert Murdoch or the Press Complaints Commission to ask them. The case for a public inquiry gets stronger and stronger.

Brian Cathcart teaches journalism at Kingston University and tweets at @BrianCathcart