Lorry drivers Ray Watts and his friend Paul Robinson had parked their trucks at the foot of the tower and were checking in with security when the helicopter struck in thick cloud.

Watts, 45, a driver for Sheffield Insulation, was delivering panels to the new building and had stopped to chat with the security guard because he hadn’t been on the site for a couple of weeks, when the debris started raining down onto his van outside.

“There was a big bang when the helicopter hit the boom of the crane and there was another when it hit the ground and blew up,” said Watts.

“I just ran. I was scared and legged it. I didn’t know which way to run because there were bits everywhere. I ran towards the station and there were still bits raining down. You come to work on a normal Wednesday morning and you don’t expect this. There were lots of workers, 20 to 25 people, waiting to get onto the site through security. They all ran in different directions. Everyone was shouting" ‘What’s going on?’”

He said he saw “a massive fireball” as the helicopter exploded and a building caught light.

He said the crane fell across Nine Elms Lane just after the traffic lights had turned red.

“They were lucky. If the lights were green the traffic would have gone through and it would have been a different story. I feel quite lucky too. If I hadn’t stopped to talk to the security guard I would have been in the truck and I’ve been told the crane hit it. I think I’ll buy a lottery ticket today.”

Robinson, 42, from Barking, was picking up waste material from the site, and was sitting in his van when he thought his friend Ray had shunted him. In fact it was metal falling from the sky onto his vehicle and he leaped from his cabin and ran.

“I saw something falling from the sky, I jumped out of the lorry and ran like hell. The boom hit the ground, I looked round and I saw the ground was on fire and somebody said it was a helicopter. We have been told the crane operator was late and was half way up. If he was all the way up he would have been hit. I felt very scared, I thought" ‘Oh my God, I am going to die here'”.

“I was shaken up for a good hour, but now I’m joking with Ray how I ran like Usain Bolt.”