As evening prayers finished at the Muslim Welfare House this weekend, worshippers spilled out on to the busy Seven Sisters Road and passed the spot where, last July, Darren Osborne mounted the pavement in a van in an attempt to kill as many Muslims as possible.

Some lingered in the drizzle, hoping to get some insight into the attack that killed Makram Ali, aged 51, and injured 12 others after Ramadan prayers at the mosque and community centre in Finsbury Park, north London. Osborne, 48, was sentenced to life for murder and attempted murder last Friday, and ordered to serve at least 43 years.

Standing against a backdrop of a union flag overlaid with a swastika, Nigel Bromage gave a genial smile as he told the Muslim Welfare House congregation and others from the local community that he was only too happy to enlighten them.

“To come here and talk to you about my past is very difficult, particularly with what happened last year,” he said. “But if I’m not honest, how will you understand what’s going on? Most people in this country equate extremism with Islam, but I’m here to tell you about the British far right and how they are organising and grooming people to become extremist and violent.”

Bromage asked the audience what thoughts the flag that opened his presentation evoked in them and was met with words such as “racist”, “Nazi” and “hate”. To shocked looks, he told them that in areas less multicultural than Finsbury Park, the response to that question was “patriotism, white pride, Great Britain”.

For more than 20 years Bromage was a leading member of the British far right and cofounded Combat 18, an openly neo-Nazi organisation that preaches a virulent, violent message of white supremacy. He also orchestrated one of the group’s most infamous acts: a riot at a football match in Dublin between the Republic of Ireland and England in 1995 as a protest against the IRA. The game was abandoned.

Bromage, now 52, became disillusioned with Combat 18 in 2000. His epiphany came when some of its members tried to throw a black man through a shop window in Birmingham and he intervened to stop it. “I realised in that moment that I was just an advocate of violence,” Bromage confessed to the gathering. “It dawned on me that in my time in the far right, I had achieved nothing apart from anger and hate, and all I’d ever done was inflict pain.”

After renouncing his beliefs, he formed Small Steps, an organisation made up of fellow reformed neo-Nazis. They now tour the country to speak about their experiences and share insights about how the far right operates today. According to Small Steps, 30-40% of members of the UK’s 30 or so extremist rightwing organisations are former or current soldiers. Instead of overt symbols, many use code numbers to identify each other. Bromage’s team have also uncovered handbooks on how to groom and radicalise potential recruits, including using teachers and social workers as a way to get to young people. Key to their work is the online world and vilifying Islam.

Bromage said: “The far right goes about its business in exactly the same way as Muslim extremists. It’s all about grooming online and radicalising people. There are many ways of doing this. You don’t have to directly tell anyone to go and commit violence, but you can inspire by inflammatory propaganda. It’s not about a street presence any more: the real focus is on the internet – social media and other forums.”

The two-week trial of Osborne heard how he became radicalised within a month of watching the BBC drama Three Girlscorrect, based on the Rochdale sexual abuse scandal involving Muslim men. After accessing online anti-Islam propaganda, he also communicated with Jayda Fransen, the deputy leader of Britain First, and regularly read online material by Tommy Robinson, the far-right former leader of the English Defence League.

According to government figures published last year, almost one-third of all referrals to the Channel programme, which is part of the Prevent strategy designed to catch people before they turn to extremist violence, was for those holding radical rightwing views. Bromage is a mentor for Channel and works with people in an attempt to de-radicalise them. The youngest person he has dealt with was aged nine, the oldest in his 70s.

Bromage is one of only a handful of people in the country doing this work, which he admits is time consuming and can take anything from three months up to a year. He is currently so inundated with referrals that he is having to turn them down.

Many of those listening to Bromage had been victims of hate crime, particularly the women in hijabs and burkas who shared their stories. His insights caused alarm and shock, but ultimately only confirmed what they and the wider Muslim community have been complaining about for some time.

Toufik Kacimi, chief executive of the Muslim Welfare House, said: “The authorities have completely underestimated those on the extreme right. Unless they are tackled with the same force that Islamic extremists are, I fear it will just be a matter of time before we see another attack like we did here.”