Five years ago today four bombers brought death and carnage to London's transport system. Those involved in a series of coincidences that helped save lives on a bombed bus explain what happened - and why they still find it hard to explain.

A scene of "utter chaos" transformed into "something resembling sense". That's how one woman describes the impact of Doctor Peter Holden's arrival at the scene of the wrecked number 30 bus in London's Tavistock Square on 7 July, 2005.

That day four suicide bombers killed 52 people and injured nearly 800. One of them, Hasib Hussain, 18, from Leeds, killed himself and 13 passengers on that bus.

Coincidence can bring mixed fortune and sometimes it can be cruel. Many passengers on the number 30 were only using it because the Tube was closed due to Hussain's three fellow bombers detonating their devices. Something they did not know as they boarded it.

I still can't explain... the series of coincidences that puts a bus on diversion where it shouldn't be, to blow up outside a building full of doctors Dr Peter Holden In depth: 7 July attacks

But chance can also bring unexpected blessings. The unfolding drama on that day meant the bus was forced to take a detour which delivered it into the hands of one of the few people in the country trained to handle a major emergency on that scale - Dr Holden.

The bus was torn apart in front of the headquarters of the British Medical Association (BMA), where he was attending committee meetings. A GP by trade, he is also trained as one of the UK's few major incident commanders.

"I thought 'I really am in it now'," he says. "Then I thought 'you've been trained for this - come on'."

By yet another coincidence, a medical conference was also being held at the BMA. It meant dozens of doctors were on hand to offer lifesaving expertise.

'Screaming and blood'

"There were more doctors there than you'd find in casualty department and certainly more senior doctors than you would find in any casualty - ever," says Doctor Laurence Buckman, who was at the conference. "There were some anaesthetists, some consultant surgeons and there were a lot of GPs."

Doctor Mandy Du Feu is in no doubt how significant this series of coincidences was. She was on another bus travelling in the opposite direction to the number 30 when it blew up.

"As I was looking at the bus coming the other way, that was when it exploded," she says . "I saw a flash, then just smoke. I just ran towards the bus instinctively. I can remember the sound of glass falling, glass breaking. It just seemed to go on forever."

Firefighter Michael Ellis was one of the first people on the scene. As he tried to help, Dr Holden introduced himself and took charge.

"It was strange, of all the places the explosion could have happened on the bus, it actually happened outside the British Medical Association," he says.

Dr Du Feu, who was also trying to help those injured, says the scene was chaotic until Dr Holden stepped in.

"Peter turned up and just assumed responsibility for everything," she says. "He just started telling people what to do and people then knew who to ask and suddenly things started clicking into place. The scene was transformed from one of utter chaos into something resembling sense."

The injured were moved into the BMA's courtyard for treatment.

Louise Barry was someone to benefit from the organisation Dr Holden bought to the scene. A commuter who had got on the bus after finding her usual Tube station shut, she lay injured in the street.

"They brought a table over, there were about six or eight gentlemen and they transported me over into the BMA courtyard using it, where there were a lot of wounded people," she says. "There was a lot of screaming, a lot of blood everywhere."

Tourniquets

But while the expertise was available, medical equipment was lacking. All they had were the office first-aid kits and London's transport system had ground to a halt, so medical supplies could not get through. It forced the doctors to improvise.

Ripped clothes were used as tourniquets, which were tightened with the help of cutlery. Anything and everything was used.

"How do you tape people's arms up if there's no tape? Well, you use firemen's duct tape," says Dr Buckman.

"There were an awful lot of doctors in the quadrangle at that point, each of them in little clusters round a patient. We were all reaching the point where people were going to die any second because we had no more stuff."

Find out more This article is based on 7/7: Saved By a Miracle? broadcast on BBC One on Monday. It can be viewed again using the iPlayer. 7/7: Saved By a Miracle?

His own patient was now unconscious and her blood pressure was dangerously low. She was very close to death. At that moment a policeman who had battled his way through the gridlock arrived with bags of intravenous fluid, which helps get blood flowing again and increases blood pressure.

"He came at just the right moment and he certainly saved a lot of lives," says Dr Buckman. "My patient regained consciousness. I don't know her name, I never saw her again, but I'm told she survived."

Finally more supplies started to arrive and the injured were taken to hospital, including Louise. But as the doctors' accounts of that day indicate, the chain of events saved many lives. It is something those involved still find incredible.

"I still can't explain... the series of coincidences that puts a bus on diversion where it shouldn't be, to blow up outside a building full of doctors," says Dr Holden.