Bogus callers posing as police officers conned a woman into handing over £30,000 worth of jewellery.
The victim was at her home in Squires Lane, Finchley, when she received a phone call from a man pretending to be PC Mason from Paddington police station.
The "officer" said the victim's bank and credit cards had been used fraudulently in central London.
He said he had arrested those responsible and found in their possession a diary of her personal details that he wanted to check and cross reference.
He requested the name on the victim's cards and read out a number of bogus sales on the card, explaining that her identity had been compromised.
The 'officer' told the woman to ring the number on the back of her bank card to report the alleged fraud.
But while the woman put the phone down to call the card number, the bogus officer did not hang up and consequently handed the phone to another man, calling himself John, who said he was employed by the woman's bank.
The woman answered all the security questions asked and gave her PIN number to the fraudsters.
"John" told the woman the arrested suspects would know where she lived and asked if she had any valuables at her home, to which she replied she had gold and silver jewellery in a safety deposit box.
The bank offered to take the contents of the box and issue her with a secure safety deposit key the following day.
A courier was despatched to the woman's address to collect the cards and safety deposit box items and was given a password for the courier to recognise.
The following day, she told the police what had happened.
Detective Constable Davina Cullen said: "These fraudsters work very hard at appearing to be genuine. The golden rule should be that if anyone even mentions your bank PIN number either at the door or on the telephone you should stop the conversation by hanging up or closing the door and call police immediately.
"No-one has the right to know your PIN number, it is private to you and a police officer would never ask you for it.
"If you recognise any of the jewellery shown in the photos, or have any information about the courier seen at 10pm on Thursday, April 3 please call 101 or Crimestoppers on 0800 555 111."
The courier is described as an Asian man between 25 and 30-years-old. He is 5ft 3ins talls, with short hair and a black moustache.