A podcast examining the unsolved murder of a private investigator, found with an axe in his head in a London pub car park, has topped the iTunes chart.

‘Untold: The Daniel Morgan Murder’, pitched as the British Serial, had been downloaded over 200,000 times by the time its third episode was released this week.

The complex, murky tale begins with 37-year-old Mr Morgan’s murder outside the Golden Lion in Sydenham in 1987 amid allegations he was preparing to expose corruption at Scotland Yard.

In the three decades since, there have been further allegations of police corruption and claims of links between his agency Southern Investigations and the now defunct News of the World.

But despite five police inquiries costing £50 million, nobody has ever been brought to justice, with the most recent prosecution collapsing in 2011.

For the victim’s brother Alastair Morgan, the 10-episode podcast, initially launched following a £10,000 crowdfunding campaign and put together by writer Peter Jukes, was the best way to get the story heard when it seemed nobody else was interested.

He told the Standard: “Peter came up with the podcast idea.

“I think the Serial podcasts had been an eye opener for a lot of people and he thought ‘why don’t we try something like that?’”

The murder is often described as London’s most notorious unsolved case, but the most common reaction from listeners, many of whom have tuned in from outside the UK, has been one of surprise that they didn’t already know about it.

Alastair Morgan, 67, said: “It’s unquestionably as important as the Stephen Lawrence case in my view but we were let down by the last Labour government. They wouldn’t do a thing about it.”

He went on: “We’re very pleased because obviously there’s a lot more people that will be learning about it who would otherwise never have done that.

“The thing is I know that most people don’t know about it whereas everyone knows about Stephen Lawrence, for good reason.

“Because of the press relationship in this case, far fewer people know about Daniel. This has been a way to try to redress the balance of that.”

An inquiry set up by Theresa May in 2013 is expected to release its long-awaited report on the case in the Autumn.

The podcast team believe they have uncovered compelling new evidence along the way. But for Daniel's brother, one of the most significant things is there will now be so many more eyes on the inquiry.

He said: “Now people do know about this because they’re watching it and are concerned about it and they’re telling people about it. That’s really the message behind the story.

“If nobody was watching, nobody was looking, then it’s kind of like ‘who cares?’”

The Islington resident said: “It’s a hell of a story, it’s a long story but absolutely fascinating.

“For me, I have to keep on and on going though it again and that’s horrible. But what I saw going on in there, I won’t be silenced and I won’t let that pass.”