The growing number of private security companies policing UK streets is a worrying development, senior police figures say...Former Metropolitan Police Commissioner Sir Ian Blair has also said there should be no role for the private sector in Britain's law enforcement... BBC correspondent Keith Doyle joined one private security company who began patrolling the streets of Darlington this week...Francis Jones of Sparta Security told our correspondent the patrols provided a visual deterrent to potential criminals...But the vice chairman of the Police Federation, which represents officers, said such firms could cause problems... "We have got people who have certain powers, we are going to see them in uniform. Potentially there is confusion there for the public and who are they actually accountable to?