Richard Madeley watched a gruesome video of innocent men being hacked to death with a machete for his latest TV project. 

Seeing the cold-blooded murders left him shaken up, but worse was to come moments later when Richard came face to face with the man who'd committed the crime.

'That level of psychotic personality comes through their eyes,' says Richard, 70. 

'I've never looked into the faces of men like that before or had eyes like that boring into mine. The atmosphere between you is heavy. That video was the most graphic thing I've ever seen. It took me quite a while to get it out of my head.'

Richard put himself through all this for his new feature-length Channel 5 documentary Richard Madeley: Inside The World's Mega Prison, which sees him given unique access to El Salvador's notorious ultra-high security jail CECOT. 

It currently houses around 15,000 inmates, many of them gang members who've terrorised the Central American country for decades.

A crackdown on the gangs by President Nayib Bukele in 2022 saw thousands of murderous thugs thrown into newly built Cecot, never to be released. 

Though the jail has been criticised for its human rights abuses, El Salvador has turned from one of the world's most dangerous countries to one of its safest, with a murder rate lower than in the US.

When Richard arrived at Cecot, he was shocked to find prisoners crammed into huge cells 'like battery chickens'. 

Sleeping on metal bunks with the lights on 24/7, the prisoners are denied any entertainment apart from the Bible, their diet is poor and they're only let out for half an hour a day for exercise and morality lessons.

'I walked down the atrium looking left and right at thousands of men, all in exactly the same conditions,' says Richard. 

'It was a major shock. My first emotion was pity – this would be their life from now on, so it's a living death. They are dead men walking.'

But Richard's questions about the treatment of these prisoners so angered the prison's director, Belarmino García, that he and his crew were ejected from Cecot after just a few hours. 

'What you don't see on camera is that once they chucked us out, I buttonholed the director and the main government publicist and said, "If I go along like a tourist being shown the sights, people will think I wasn't objective. For you to be able to defend this prison, I've got to put hard questions to you." They slept on that and then let us in the next morning, but it was tricky.'

Although he wasn't meant to speak to the inmates on his return, eventually the director allowed Richard a five-minute talk with a prisoner known as Psycho, not the machete murderer he'd seen the day before but another violent gang member. 

'What I got from him was a sense that he'd accepted the hopelessness of his position,' says Richard. 'He was king of the world at one point and now he's paying the bill for that.'

Richard also meets Salvadorans who are jubilant at being freed from gang violence. 

'If gang members needed money, they'd kidnap people off the street,' explains Richard. 'They'd demand money for their release and if they weren't released within a day, they killed them. So ordinary people were being terrorised. Now they're free.'

The documentary shows the Good Morning Britain host still at the top of his journalistic game and, impressively, it airs just days after he turned 70. 

'It doesn't feel like a new string to my bow,' he says. 'It feels like I'm going back to my roots, because this is how I started in the profession over 50 years ago. 

'I spent the first 20 years of my professional life as a reporter and I absolutely loved it. So it's been good to see that age and experience can still count for something in my game.

'Age gives you perspective. For example, I'm old enough to remember my parents' tears when Kennedy was assassinated, and England winning the World Cup. And after over 50 years asking questions and reporting stories, hopefully I've learned a little along the way.'

He hopes to be asked to do more documentaries, though he still loves the day job. 'GMB is a really tight, upbeat and professional outfit,' he says. 

'Susanna Reid is probably the most professional journalist I've ever worked with. Her attention to detail is phenomenal.'

He's got a podcast with his daughter, fitness trainer Chloe Madeley, in the works, and still runs the Richard and Judy Book Club with his wife Judy Finnigan, with whom he presented This Morning from 1988 to 2001. He's also hit on a novel way to hold back the advancing years. 

'Sir Trevor Nunn, who's 86, says that since he turned 70, he started counting backwards on birthdays. So that's what I'm going to do!'

Richard Madeley: Inside The World’s Mega Prison, Wednesday, 9pm, Channel 5