A decision by the CPS not to prosecute does not conclusively determine that there is insufficient evidence or a that prosecution is not in the public interest. Private prosecutors are therefore generally free to start a private prosecution after a decision by the police or CPS not to prosecute or to stop a prosecution. Indeed, private prosecutions are themselves seen as a valuable safeguard against mistakes, inertia or misbehaviour by the official prosecuting authorities. The CPS will not, therefore, take over a private prosecution simply because it was started after the police had decided not to prosecute, or the CPS had stopped its prosecution; the Supreme Court has confirmed that private prosecutions remain possible in appropriate cases even where the CPS has decided against instituting a prosecution.
That said, if the CPS receives a request to intervene in a private prosecution, for example from the defence, then it will consider whether the Full Code Test is met. In the event that it is not met, the CPS should take over the prosecution and discontinue it.