Since the murder of Stephen Lawrence 25 years ago – through the succession of botched police investigations and the landmark Macpherson report in 1999 – those of us closest to the unfolding events have developed a set of certainties. We knew all about the racism and incompetence that let Stephen’s killers walk free for so long. We knew all about the intransigence and denial that led the authorities to close ranks, even in the incontrovertible face of wrongdoing.

But by the end of this painful, difficult period, with two of the killers convicted, a whole series of policing improvements enacted and reassurances that the mindset of our police service had irrevocably changed, we also felt certain that good had sprung from tragedy. That the kind of police officer who might have doubted that racism was a problem, or doubted that the Metropolitan police service’s institutional failings explained the shameful treatment of the Lawrences, would now be aware. If Stephen was to have a lasting legacy, that, we felt, was part of it.

The cases we are getting involving racism and the police tell you the mindset is not much changed since Stephen’s death

So imagine the scene just a few days ago, when Doreen Lawrence and I sat in a small screening theatre in central London to watch the second part of the BBC documentary Stephen: The Murder That Changed a Nation. This episode, broadcast this evening, focused on the police officers most intimately involved with the Lawrences around the time of their private prosecution against the suspected killers in 1996. These were officers with whom we had had a distant relationship at first, but who over a period of time won our confidence. We came to believe that they really wanted to bring Stephen’s killers to justice; but more than that, we believed they really had been changed by the experience of knowing how badly their institution had treated a vulnerable family at their direst time of need. We did not hold back from them. They seemed to feel what we felt. They knew what we knew.

The documentary features Bill Mellish, who is now retired but as a detective superintendent was deployed by the Met as senior investigating officer at the time of the private prosecution. He told the programme: “I think one officer on the night [of the murder] formed, wrongly, the opinion that it was a drug-related murder.” But watching the broadcast we were aghast when he continued: “And I think on that basis and perhaps one or two other examples [they] branded the whole of the Metropolitan police as racist. Utter rubbish.” Asked for his opinion of the Macpherson report, a process many feel contributed to a sea change in Britain’s thinking on policing and race relations, Mellish said: “It confirmed my suspicion that it was a kangaroo court. I know they’re wrong. I have been in the Met for 32 years.”

He went on to attack Lady Lawrence as ungrateful, saying she used as a gimmick a tendency to never smile; and that I and Michael Mansfield, the family’s barrister, were extreme leftwingers working to our own agenda.

Hearing those words felt like an almighty kick in the gut. I felt betrayed. I looked at Doreen; she gave a sharp intake of breath, then fell silent. It is hard to describe the shock. By the end of the episode we were both shaking with fury. We had assumed that Mellish, like us, had been enlightened by the Macpherson revelations. But, having left the Met, here he was, free to say what he really thought.

And the meaning of what he said was clear to both of us. If he, who has had 25 years to dissect it all, could still dismiss what he saw in this way, what about those who were not that close to it, or had never met us, and those who are there now? If we could not convince him, what chance do we have with the others? It seems to me now that most of what the police did they did for themselves, not the family. It was basically the Met’s crisis management.

Of course, Mellish is just one former officer. In the years that have passed, many senior officers have said positive things about the impact of the case on how they police communities. But my fear is that much of what they actually learned was how to spin things better. And of course we now know that while the Met should have been helping the Lawrences it was simultaneously spying on them, using an undercover officer.

The most worrying thing for me is that this chimes with what I see right now in my day-to-day work as a lawyer. The cases we are getting involving racism and the police tell you that, for too many, the mindset is not much changed since the time of Stephen’s death. We have hundreds of cases of police officers acting in a terrible fashion.

You cannot help but think that the improvements we so wanted to see were only skin deep. Maybe we fooled ourselves into believing they were really happening. I am left wondering how much of the Met’s latest advocacy of stop and search is about stopping violence on the streets, and how much is just about getting back to the bad old days. The job for Cressida Dick, the current Met commissioner, is to prove her intentions are genuine.

But we must not lose hope. Society has moved on to the point where there is an acceptance that racism exists. There is legislation that at least sets down how the police should behave. And though the possibility of further convictions in the Lawrence case itself is now remote, we can, begrudgingly, accept that.

So what remains is to remember Stephen, and what the Lawrences have done for this country, but also to learn the lessons his tragic death taught us. I am not at all sure that is happening at present. The police reflect society; if there is to be racial justice, they must play a part. But with his words, one former detective has unwittingly revealed the task the commissioner faces, and how much further she still has to go.

• Imran Khan is the lawyer for Stephen Lawrence’s family