Warning: contains spoilers

In Sunday’s Bodyguard finale, our hero Sgt David Budd woke up in the middle of London wearing a suicide belt operated by a “dead man’s switch”. Were he to lift his thumb, the bomb would explode killing everyone in the surrounding area. It was a nightmarish scenario. But how realistic was it?

The Telegraph asked a leading security specialist, a former crisis negotiator and Britain’s first female bomb disposal officer how they would handle the situation – and each of their responses was revealing.

The first response: how the police would mobilise

The police were quick to lead Budd to a more isolated area, but saving his life was low on their list of priorities. This part rang true, according to intelligence expert Dr Kristian Gustafson. “You’ve got to pretty much presume that the man with the bomb on him is a dead man.”

Dr Gustafson, formerly senior lecturer in War Studies at Sandhurst and an ex-Canadian Army officer, is now deputy director of the Brunel Centre for Intelligence and Security Studies.

“He’s got a bomb on him - that’s not coming home,” he continues. “You’ve got to assume that it could go off any second, so you mitigate hard: clear the area, contain it.

“Only once everyone else is safe, and the harm is minimised to the guy wearing the device, that’s when you’d call in the ammunition technical officers or ATOs – the bomb guys – to defuse it.”

In 2003, the Metropolitan Police introduced a policy called Operation Kratos: officers in an armed, hostile encounter with suspected suicide bombers were advised to shoot to kill, aiming for the head.

The type of explosive makes no difference – it’s motivation that matters. “If a guy’s hostile with an IED [improvised explosive device], you kill him, whether it’s a dead man’s switch or not,” says Gustafson. “But if you’ve got someone going ‘I’m trapped in this bomb, I don’t want to do this!’ that’s a different scenario. One question then would be, is he trying to lure in the ATOs?”

According to Gustafson, terrorists sometimes use misleading tactics to target the first responders. “Part of the reason why you do certain kinds of attacks is to see how the security services respond. This is something the IRA did.

“You set up small devices and hurt some people, then the police and the ambulance come – and you’ve attracted more high profile targets to the scene. If you know the radiuses, how and where they set up their headquarters, you can site a secondary device to kill them.”

Ultimately, the key decisions would not be made by the officers on the scene. “The British police have a chain of bronze, silver and gold command,” Gustafson explains.

“Bronze is the person on the spot. People don’t just wander up and say ‘I’m the ambulance guy, can I help?’ They all go through the silver commander, who manages the resources. The bronze commander only has one radio line open, and that’s to silver. Bronze might say ‘I need ATOs, and I need to block off these streets.’ Silver will make it happen.

“There will also always be a designated gold commander above them: in a scenario like this it could be the chief of the Met. The bronze commander might ask ‘Can I shoot this guy, or do we want to talk to him and try to defuse it?’ It’s the gold commander who decides whether to authorise lethal force.”

The negotiation: why you (probably) shouldn’t swear at a bomber

Cathy MacDonald is a former crisis negotiator who spent more than 30 years in the Scottish police. She’s also a Bodyguard fan, having been won over by a scene in the first episode where Budd convinced a suicide bomber to back down.

“I was impressed with what he did in that situation,” says MacDonald. “A lot of reassurance, a lot of connection. He recognised she was isolated and needed help. I thought the writers had obviously been quite well advised.”

But if Budd was a good example of how to speak to someone with their finger on a red button, the police officers who confront him in the finale were a textbook example of what not to do.

“The characters Lorraine Craddock and Louise Rayburn were too wrapped in their own thoughts, judgements and opinions,” says MacDonald. “Budd was the one who was showing heartfelt and clear communication - but they weren’t listening.

“When you’re talking to someone in a personal crisis it’s really important that you’re nonjudgemental. A lot of people we talk to feel they’ve never been properly listened to in their entire life.

“If you’re trying to explain something that matters a great deal to you, and the other person isn’t listening, it creates frustration and annoyance. The one thing I would never do when speaking to a person in crisis is say something that might spark an unhelpful emotion.”

Things got worse when they brought another voice into the conversation. “The arrival of his wife was so flawed and high risk it had to be for dramatic effect only.” Meanwhile, the police in the scene actually stoked Budd’s frustration, dismissing what he said. One even shouted at him, saying "It's been a stream of bulls--t since day one!"

Is that kind of language in a negotiation ever a good idea? Not usually, unless you’re trying to establish a rapport with someone who’s already swearing casually. “Sometimes you have to match language codes. The pace and tone of the way you speak can be more important than the words you say.”

But aggressive cursing - even when you think you’re being lied to - is a mistake.  “Sometimes you have to give them a reality check, but it’s never confrontational and never in a way that would make them react emotionally.”

MacDonald is hesitant to reveal too many of her techniques. “It’s a really covert world, and we don’t share a lot of what we do for a very good reason. Although I’m now retired from that [she now runs a company called Art of Communication], I would hate to compromise my colleagues.”

But MacDonald agreed to share one rare example of a successful use of foul language. “One relevant scenario I’ve had was not terrorist-related, but human behaviour is human behaviour. It was a suicide intervention. Our conversation had been polite and respectful, but  where I genuinely believed they were about to jump.”

“Sometimes, they’re so focussed and emotional that they can’t really listen to somebody. So I’d shout in a way I normally wouldn’t, like a pin-prick bursting the bubble, just to make them realise I’m there. In that case, I yelled, ‘For f---’s sake!’ It was so alien to the language we were using that they were like, ‘Woah!’ It was enough to break the tension.’”

Moments like that are the exception, not the rule. “If someone is in personal crisis, the specific scenario doesn’t matter: you always have to listen and try to understand their world.”

Defusing the bomb: don’t get your wires in a twist

Lucy Lewis was the first British woman to work as a bomb disposal officer. “It’s a rule that you never do the same thing twice,” she says. “The vest bomb in the first episode was dealt with by cutting the straps, and the one in the finale was made by the same bomb-maker - but they had booby-trapped the straps.”

It was a believable development, she explains. “It’s always an arms race. If they realise you’re using a metal detector, they’ll make it out of wood.” Though Lewis never worked in this kind of high-threat situation, she has first hand experience of defusing Second World War bombs. The Germans used similar tactics, she tells me; after learning that the British were pulling out their fuses, from around 1940 they started placing anti-withdrawal traps underneath.

Lewis currently works as Cambridge University’s marshal - effectively the head of its ancient private police force - and is the first woman ever to hold the post. It’s not her first record: in 1990 she became the first female bomb disposal operative to be used for a mission.

“I was involved in Operation Crabstick,” she says. “Over a weekend, we cleared 26 pipe mines from under Eastleigh Airport.” The airport and the surrounding fields were sealed off, with only one civilian let through each day: an 88-year-old farmer who needed to feed his pigs.

“There are still quite a lot of bombs from the war around,” says Lewis. “Every time we have a drought or a heatwave they crop up. We’re still doing area clearances. And there’s always a grenade in granny’s attic.”

Lewis is often bothered by Hollywood clichés about her line of work. She gave up on The English Patient when she reached its famous wire-cutting scene. “I got very shouty at it. It’s an iron bomb!”

What a real operative would do with that kind of bomb, she explains, is drill a hole in it and use a bicycle pump to fill the fuse with a pressurised salt solution. “That neutralises all the crystals, so we can then cut the entire fuse pocket out, then make more holes and steam the explosive out. The whole idea of cutting wires? They didn’t cut wires in 1940, let alone now.”

Lewis says Bodyguard’s bomb disposal operative made a few rookie mistakes. “The operator would not have gone straight for the bright shiny screws on the cover, as these look like a come-on.” After all, you never know which part might be booby-trapped.

“He would have used a robot to do the initial recce, and not have taken two team members that close - particularly as he had complete freedom to move round Budd and good visuals.  He would not have made Budd turn round, but would have circled him.”

But Bodyguard was still better than most. “Overall, it was one of the better on-screen bomb disposal scenes I've seen - I could enjoy the plot without getting overly annoyed by the obvious errors.”