Michael Gavin saw the accident while he was standing at Vauxhall station.
I was facing the building when it happened and I heard a loud bang. The top of the crane was actually obscured by fog so I didn't see the impact.
But I heard a bang and saw the body of the helicopter falling to the ground along with pieces of the crane and then a large plume of smoke afterwards.
More top news
A construction worker was crushed when a platform collapsed on them while the set was being dismantled.
Nasa's Juno spacecraft will get closer to the giant planet than any other spacecraft before.
Theresa May may be able to trigger withdrawal from the European Union without parliamentary approval.