The Jihadis Next Door: Documentary tracks bouncy castle salesman Abu Rumaysah's journey to the heart of Isis

“It’s a bit of a mess but I’ll show you,” says Abu Rumaysah, the British man suspected of being the masked militant in the latest gruesome Isis execution video, as he rummages around his garage in Walthamstow, east London.

The former bouncy castle salesman emerges triumphant. “These are the black flags of Islam. This one’s actually the flag of the Islamic State, so one day when the sharia comes, you will see this black flag everywhere,” he proclaims, vowing that the flag will soon fly over Downing Street.

The extraordinary footage of Abu Rumaysah, who fled the UK to join Isis in 2014 having previously been arrested six times, was shot by the film-maker Jamie Roberts for a Channel 4 documentary, The Jihadis Next Door, screened on Tuesday night.

Roberts spent two years filming a group of Islamist extremists in London, including Abu Rumaysah, last arrested as part of an investigation into alleged support for the banned extremist group al-Muhajiroun. He left Britain with his wife and four children the day after being released on bail, travelling to Paris and then Syria.

Channel 4 has declined a Metropolitan Police request for a pre-broadcast viewing of the film, in which two other activists already known to the authorities, Mohammed Shamsuddin and Abu Haleema, laugh while watching an Isis murder video and speak of recruiting fellow British Muslims through “brain-washing”.

Abu Rumaysah, real-name Siddhartha Dhar, has not been officially confirmed as the masked figure in the video, released a couple of weeks ago, which shows the murder of five men accused by Isis of spying for the UK.

When the video emerged, Roberts received a text from Shamsuddin, which had a link to the video, and a message saying: “You may know the voice.” Roberts said: “The voice instantly gave me chills. He has lost weight and it may not be him, but I felt like I recognised his voice.”

In the film, Abu Rumaysah tells Roberts that he is from a Hindu background and converted to Islam when he was about 19. “One man died in Woolwich, Lee Rigby, and the whole country went up in uproar,” Abu Rumaysah says. “There are many Lee Rigbys in Muslim countries, and if these issues aren’t addressed, we can expect more carnage in this country and more cycle of violence.”

He tells Roberts that the notion that the black flag of Islam would one day fly over Downing Street may have been described as “ludicrous” 10 or 15 years ago. But he says it is now “a very real possibility the way Muslims are coming forward in this country. We don’t believe that authority should be in the hands of the non-Muslims”.

Describing Abu Rumaysah as the “dullest and most one-dimensional” of his subjects, who spouted extremist ideology without any hint of outside interests, Roberts found it difficult to comprehend his making “a journey of that magnitude” to the Syrian battlefield.

Haleema, a radical preacher who has had his passport removed and is banned from using social media to promote his views, said he last saw Abu Rumaysah citing the Koran in prison. “The guy was just turbo-charged, always on the go,” he said.

Shamsuddin, who says he knows several people who have gone to fight in Syria, and Haleema support the declaration of a caliphate, but refuse to say on camera that they support Isis’s aims and methods, fearing that they would “get nicked” – support for Isis carries a possible six-year jail sentence. The film shows the extremists being confronted frequently during street demonstrations by moderate British Muslims who reject their philosophy.

Haleema talks about grooming his beard with oil. Shamsuddin tells Roberts he is a fan of The Great British Bake Off although he no longer considered the 2015 winner, Nadiya Hussain, to be a Muslim.

‘The Jihadis Next Door’, Channel 4, 9pm Tuesday 19 January