By Martin HickmanTwo mystery cars were conducting surveillance of Chelsea Harbour around the time Rebekah Brooks was arrested for phone hacking, according to a counter-surveillance expert working for Rupert Murdoch's newspapers.Tony Cox, a director of Corunna Solutions Ltd, told police that when Mrs Brooks was released from Lewisham police station on the night of 17 July 2011 the riverside complex was being watched by a Silver Audi A3 and a gun-metal grey Astra.Mr Cox, a counter-surveillance expert working for News International, gave no further information about the vehicles in a witness statement he gave to the Metropolitan Police on 13 March 2012.The statement was read out at the phone hacking trial at the Old Bailey by chief prosecutor Andrew Edis QC.In it, Mr Cox said Corunna Solutions had been sub-contracted by another security firm contracted to NI, International Corporate Protection, to help protect Mrs Brooks, who was given the codename Black Hawk.Corunna's role included carrying out conduct counter-surveillance to protect Mrs Brooks from "hostile" journalists and members of the public.Mr Cox said that as well as the two mystery vehicles, another two cars were surveilling Chelsea Harbour around the time of Mrs Brooks's arrest: a black Hackney carriage and a Volkswagen Golf belonging to Corunna Solutions.He told detectives that Mrs Brooks had returned to Chelsea Harbour, where she lived with her racehorse trainer husband Charlie, in a black Audi A8 shortly after midnight on 18 July 2011.Mrs Brooks, Mr Brooks and News International's group director of security, Mark Hanna, deny hiding evidence from detectives as part of a conspiracy to pervert the course of justice.The trial continues.