On a desk in the centre of the room, a battered Nokia phone rings. It is known as the "HAT phone", and the call signals only one thing: a suspected murder has occurred.

It is a Friday afternoon and the Sutton murder squad – one of 24 homicide teams in the Metropolitan Police – is on call.

The squad gathers in their incident room, a large open-plan office with mugshots and crime scene pictures on the walls and witness statements littering the desks.

The initial information is that a man has been found face down on his bed with a wound on his head. Nearby is a blood-stained mop and bucket.

The Homicide Assessment Team (HAT) – three detectives whose job it is to attend murder scenes first – are dispatched to the sheltered housing complex in Brockley, south London.

Hours later their boss, Detective Inspector Chris Le Pere, follows. Stepping inside the run-down building he walks through a narrow corridor, pausing only briefly to nod hello to two PCs standing guard outside a door. On the other side of the door lies the body of Alan Bradley slumped across his bed.

After a cursory greeting to the uniformed officers, DI Le Pere steps into the accommodation's dining room: a small space dominated by a large dining table and armchairs that have seen better days.

For the next few hours the dining room will become the MIT9 briefing room inside which Le Pere and his three homicide detectives will discuss the case with two borough detectives and their DI.

What they discover will determine which of the teams takes the job. If it appears to be a murder the homicide team will investigate. If not, the borough detectives will keep it. "Right, what do we know?" asks Le Pere.

He is told that Alan Bradley, a 55-year-old alcoholic who suffered from schizophrenia, was found face-down on his bed by a member of staff at 1.40pm that day. The mop and bucket is in the kitchen of the three-bedroom flat with blood on the handle. "He has a deep wound above his eye," DS Kate Wilson says.

Bradley's two flatmates – James and David – are in custody. They are being treated as witnesses, but one of their stories doesn't seem to add up.

In his initial interview, James said that Bradley had fallen over the night before and cut his head. But he has neglected to tell officers some details which they already know – namely that he had an argument with the victim shortly before he died – and it has aroused their suspicions. Also his suggestion that the cut happened the night before is weakened by the fact that the wound is very deep, but there is no blood around the building.

It is looking like a murder case and James appears to be the main suspect. DI Le Pere is anxious that everything is done properly. "Have we done house-to-house enquiries?" he asks.

"Yes, boss. Nothing much came of that," a voice answers.

"Has anyone spoken to the family? Have we searched the rooms for a potential weapon? Have we seized the CCTV? Do our witnesses have mobile phones? Have we seized them? Has someone spoken to the sniffer dog people? How are we going to secure the scene over the weekend?"

If the man has been murdered Le Pere's team will investigate. It could take them hours, days or weeks to solve. Sometimes a perpetrator is never caught. "There is nothing worse than an unsolved murder," DI Le Pere says. "You never like to think that anyone is clever enough to get away with it. That is not an ego thing, but you would like to think that an experienced murder team would be able to find the shreds of evidence we need to charge someone."

The MIT9 team has 30 detectives who each work different shifts. But on an on-call week they will all be required to leave their beds and work through the night should a murder occur. It's for that reason that DI Le Pere keeps a camp bed under his desk.

"There have been occasions where, on a big job, I've found myself finishing at 6am and if I'm due back on at 8am there's no point going home. But I've done my fair share of nights," he adds. "It's a young man's game so I leave it to the others now."

One of those is Detective Constable Danny Chatfield. A former journalist, he decided to join the force in July 2003 after being inspired by an Old Bailey case he covered in his previous life as a reporter for Central News Agency.

"It sounds really sad," he says. "But I genuinely did get a calling. I was covering the case of an immigrant who had been murdered. The bloke had no family and not many friends and the only people who cared about his death were the detectives sent to investigate it. They got justice for him when no-one else was bothered and it really inspired me."

Court reporter Chatfield soon became Constable Chatfield. After serving two years in uniform he passed his detective's exam and joined CID in 2005 and transferred to Homicide in May last year.

He has been with MIT9 for nine months and as one of the newer members of the team he is still getting used to the social havoc that is the night shift. "Before I came to the murder squad I had been with my girlfriend for 11 years," he explains. "Within a few months of coming here the relationship had finished. I can't blame that solely on the job, but it does play havoc with your social life.

"When you are on call you certainly do not make any plans because you could get a call at any time. You can't have a drink either so it's probably the healthiest week I do. I suppose it's a shame it's only every eight weeks.

"You can't complain though. As a police officer I think working murder is the elite. It is hard work to get chosen for this so while it might sound bad at times I wouldn't swap it."

Back in the briefing room one of the detectives gets a phone call. The voice on the other end tells him that James has now clarified his story and included the fact that the pair had had an argument.

The spotlight of suspicion on James dims further when the crime scene investigator says that the congealed blood around the wound above Bradley's eye indicates the injury could well have happened the night before. Why there is no blood on the floor remains a mystery although a bloodstained rag on a radiator suggests the victim had attempted to treat himself.

It is looking like an accidental death but when the briefing finishes one of the borough detectives is keen to uncover the perpetrator of another murder. "Who killed Archie?" she asks. "I've missed EastEnders."

While the case of Archie Mitchell is one that has obviously gripped this particular detective, do the real cases keep the detectives awake at night, wondering who the killer is?

"No," says DI Le Pere. "This job can be tragic at times. But you need to be able to switch off. I have a family. When I go home I don't want to talk about work. Not only is it unprofessional, but they don't want to hear about it.

"My wife might know I am dealing with a certain shooting or stabbing, but she will not ask about the ins and outs and I won't talk about them. I'm happy with that because the last thing I want to do when I go home to my family is talk about murders over dinner."

It is nearly 11pm as DI Le Pere leaves to go home. The Alan Bradley job is the closest his team came to a murder in their week on call – a rarity in a city which averages more than two murders a week and has just three teams on call at any one time.

Yet it is treated like a murder right up until the moment a coroner says otherwise two days later. As well as the various degrees of murders the team has to investigate – from category C domestics to Cat A whodunnits – Le Pere's team, like all other homicide squads in the capital, also have to attend any suspicious death, just in case it turns out to be murder.

But, he explains: "Murder investigation is feast and famine. It could be absolutely mad, where you don't sleep for the first 48 to 96 hours in a big case and the intensity can last for weeks until you charge someone. But then it goes back to eight-hour days where people will be waiting for the next job.

"It's a strange one because while we always have work to be getting on with, we are employed to solve murders and so we like it when we get a new job because that is what we excel at. That's not to say we sit around hoping people will die. I would never wish for anyone to be killed and every murder is a tragedy, but sadly in a city as big as London, there are always going to be murders and you are always going to need people to solve them – that's where we come in."

The MIT9 team: Experience counts in big cases

Between them, the detectives of MIT9 have a wealth of experience in big murder cases. It was they who investigated the murder of Sally Anne Bowman, the 18-year-old who was murdered, raped and robbed outside her home in Croydon in 2005. Mark Dixie, a pub chef, was convicted of her murder in 2008.

The squad's Detective Chief Inspector Nick Scola investigated the murder of Nisha Patel-Nasri, a Metropolitan Police special constable who was murdered by her husband Fadi Nasri in May 2006. Nasri was convicted in 2008.

And Detective Sergeant Eric Sword worked on the case of the television presenter Jill Dando. Barry George was imprisoned for her murder but was subsequently freed on appeal. The case remains open. The team also secured the conviction of Mario Celaire in 2009, who, following the abolition of the double jeopardy rule, became the first person in Britain to be convicted of a crime for which he was previously found not guilty.