A very posh lawyer and I would argue when, exactly, I received the message that something big was happening.

By the time I was being cross-examined in the Old Bailey, a year later, I wasn’t sure it mattered any more. It was fact. A British neo-Nazi had admitted planning to murder his MP. Indeed, he was only days away from doing it.

Someone who was in the room when the killing plan was announced, texted me. I was on holiday. My break was ruined. I’d spend the remaining days abroad hanging by the telephone making hushed calls, answering questions and offering reassurances.

I no longer made phone calls without encryption. Even we, the old dogs of Hope Not Hate, were being taught new tricks by the modern breed of Nazis we were monitoring. They were also hunting us and were sophisticated at hiding themselves. They bragged they were impenetrable. Only an act of gross stupidity on their own part could catch them out.

We were in the very guts of National Action, an outlawed organisation concerned with terrorism. If members were caught, they faced up to ten years’ imprisonment.

We discovered not only had National Action members ignored a government ban; they’d upped their game, changing leaders and opening a gym exclusively for training members and supporters. They not only wanted a race war, they were preparing for one.

We blogged on our website, exposing titbits of information we’d uncovered about the group, making little hints and digs at them, letting them know we were still watching them. But it wasn’t until one of their most senior members came to us that we had our eyes fully opened to the scale of their operation and just how – no matter how experienced we were with dealing with the most hardline of neo-Nazis – these kids were deadly serious about terrorism.

They idolised terrorism for the sake of it. Academics had focused on their aesthetics, trying to find ideological justifications for their nonsense, playing into the hands of a small but sinister clique desperate to be feared as brilliant minds. They churned out dross after dross until even they tired of themselves. They became vulgar and ‘brutes’.

Across the political spectrum from Timothy McVeigh and Anders Breivik to Jihadi John via the Provisional IRA and Khmer Rouge, National Action abandoned its own ideology to admire others, seeking to mimic the very best at doing the very worst.

Their degeneration terrified us. National Action members existed inside their own cult world, cut off from the rest of society, communicating only with each other where possible. They, themselves, often didn’t know who they were talking to; only by their individual quirks and perversions would they learn the identities of people in their new, secret chat groups.

Like any group, National Action had its schisms and obtuse personalities, but more than any far-right organisation we’d come across, this motley crew of sad individuals egged each other on, driving their racial hatred and perversions to new levels. They were fascinated by pornography, beheadings and violence.

National Action operated under intense pressure and paranoia because members knew they faced up to ten years in prison if they were they caught still active, still recruiting. They no longer wanted simply to shock and offend little old ladies and Daily Mail readers. Left to their own devices, packaged away in darkness, racism, fascism and Nazism were no longer enough for them.

Although National Action attracted many young men through its bright graphics and a sense of danger, it explored ideas and ideals that went as far as embracing Satanism. Throughout NA’s existence, both publicly and privately, rape and paedophilia were often seen as weapons to use on or against their enemies.

In Robbie Mullen we found an individual who was bright and fiercely independent but unable to extricate himself from National Action through fear and habit. Seeing inside NA through his eyes, we understood how difficult it was for him to leave.

From the moment we received his unsolicited email begging for help, he impressed upon us how urgent it was, how imminently National Action was preparing to unleash terrorism and violence. Mullen warned us that although we’d been close to understanding the real nature and truth about [the group], we’d no idea how ready and desperate this group was to kill people. The clock was ticking down to murder.

Their outbreak of terror was to be the ‘White Jihad’. More than fascism, or the usual drunkard’s admiration of Hitler and the smashing of a few shop windows and spraying swastikas on walls, by the time Mullen came to us National Action was at the point of no return. It felt defeated, there was nothing left for its members to do but destroy themselves and those around them in the desperate hope others would follow suit and there’d be an almost apocalyptic ending of society.

The national outpouring of grief after Jo Cox was murdered in June 2016 excited National Action. A vile tweet, glorifying in her killing, led to the group being proscribed by the government. The following year, like ghouls, NA members travelled to Manchester Arena to watch people grieving after a jihadi suicide attack.

Death and terror had become their only obsessions. They wanted others to suffer in similar sadistic acts of their own doing. Their new leader wrote copiously about how he was a medieval king destined to rule a dystopian society.

Mullen contacted us because he’d wanted us to break them up, where the government and the police had failed. National Action obsessed over every sentence we wrote about them, so it seemed only natural that we should be the ones to bring them down. Mullen had realised all the violent things he’d heard from NA members were more than just words. He’d realised the bloody training they undertook in their gym and elsewhere was to prepare his comrades for murder.

It came to a head in July 2017, when a National Action supporter one evening in a Warrington pub in July 2017, when National Action’s leader was told by one of their supporters he was going to kill the MP Rosie Cooper and, if he could, a woman police officer investigating him for grooming boys.

Mullen panicked, leaving the pub and texting me, alerting me to the impending bloodbath. I heard my phone beep, but I was in bed and ignored it.

I rang him back first thing the following morning. Armed with Hope Not Hate’s assurances and our personal fledgling friendship, our rescue plan swung into action. Nick Lowles made the relevant phone call to the authorities while I calmed down Mullen who was to say the least, anxious, terrified and confused.

But first things first, I asked. Who even was Rosie Cooper?

Matthew Collins is head of intelligence at Hope Not Hate. Nazi Terrorist: The Story of National Action is available to buy now