If an intruder is spotted by radar, the controller tells the Arrows in terms of bearing and range from their datum and one of the 'singletons' usually breaks off to identify it. Any type of radar service provided is dependent on the radar unit having a 'spare' ie un-used frequency available to provide an 'exclusive' service with no other traffic on the frequency. I did it many times from Farnborough. Even a helicopter air taxying at the airfield where they're displaying will cause them to break off their display