In September 2013, veteran doc maker Fergus O’Brien took up a new post as Executive Producer at the BBC, working with head of documentaries Ayesha Rafaele. On his first day, he was handed a very big project: “Literally I was walking in the door and I bumped into Ayesha and she said ‘Do you fancy exec-ing the Met?’ I’m not sure I knew exactly what that would mean but I said yes.”

O’Brien soon found himself immersed in steering one of the biggest access-driven documentary series in the BBC’s recent history. Airing on BBC One, The Met: Policing London is the first time a broadcaster has been given comprehensive access to London’s police force.

For O’Brien, it has been rather a bumpy ride: “Inevitably with stuff that’s dealing with the law and criminality and so on, the phone never stopped. You’re often managing people’s worries, and people’s concerns, and keeping an eye on the legalities of things and keeping a steady line of contact open with our editorial policy team and our legal team.”

Initially a six part series, the team had to drop one of the episodes, when legal restrictions prevented them from airing a major storyline about domestic abuse: “That was very difficult – it’s hard to say goodbye,” says O’Brien. “It would have been a really strong story and often those stories, where the victim is willing to be on camera, aren’t told. Unfortunately through the peculiarities of the legal system we couldn’t show it.”

As director of such films as Channel 4’s Seven Days and the acclaimed, and very funny, The Armstrongs (BBC One) O’Brien is used to following a variety of strong characters across numerous settings. But helping the four shooting teams negotiate their way through the labyrinthine Met was a job like no other: “Each of the teams was assigned to a response team in a different borough of London, and a cross-section of boroughs which would reflect the diversity of the city,” he says. “And each team also took on one or two specialist units, whether it was homicide or Trident. The idea being that the bigger units would hopefully provide us with a spine for each film and something we could come back to, and then we could pepper it with a mixture of different response stories to flesh things out and give a sense of variety in each programme.”

Whilst access had been given from the top, it continued to have to be negotiated throughout: “We had to get consent from everyone, even if they were in the background,” says O’Brien. “It’s the usual thing, from our point of view: unless people want to do it there isn’t a point. If they feel they are being forced into it, it just isn’t that great.”

The production ended up with 2,000 hours of footage, shot over the year, and edited over many months. Even now, as the series is airing, O’Brien is still putting out fires: “It’s not the same as covering a story and when it is done and dusted in the courts you put it out. It is just ongoing; it’s daily. Every day now we have to check every single case across the series to make sure people haven’t re-offended and we’re not in contempt of court. It’s a huge part of it.”

The Met: Policing London is airing on Mondays on BBC One at 9pm until early July. Read about a very different way to film police docs here.