North London
The two women and one man arrested knew the deceased, police said.
The officers were responding to reports of a fight.
The woman's injuries are not life threatening.
An 11-year-old initially arrested was released with no further action.
A battery short circuit is thought to be the cause.
'They just rode up, grabbed it and zoomed off.'
Party claims 13,000 people bought tickets for the event in north London.
Unite general secretary speaking at Labour's music festival in north London.
Eight fire engines called to fire.