The image shows one of two men who posed as a policeman to kidnap the wife and eight-year-old son of depot manager Colin Dixon after calling at the family's home in Herne Bay, Kent, England.

Kent police said the man, wearing dark police-style clothing and possibly a wig and false moustache, deceived Lynn Dixon into thinking her husband had been involved in an accident.

He and another man then forced Mrs Dixon and her son Craig into a large saloon car and drove away.

They were later threatened as part of the record heist on a cash-storage depot, which could have netted the robbers up to £50m ( 70m).

The Dixons' eldest son spoke out yesterday about the family's ordeal. Dominic Dixon, 21, said that the thieves should have "guns held to their heads to see how they like it".

The skip-hire worker, who has been unable to speak to his parents due to tight security surrounding the couple, also told how his little brother Craig, who turned nine yesterday, had an amazing memory, and may well recall the faces of the kidnappers.

Dominic said: "Craig's a very observant little lad. He's got an amazing memory too, he remembers things no one else remembers. If he had seen one of the robbers he would remember their face."

Police forensic experts are examining bags of cash recovered from a white Transit van, spotted by a member of the public in the car park of a hotel in Ashford.

Yesterday Kent police said Colin Dixon set off from the secure compound for his home at 5.30pm on Tuesday.

He was pulled over by a Volvo car with blue flashing lights and approached by two men wearing high visibility jackets and police-style caps, who said they were officers.

He was handcuffed and driven off to a building, possibly a farm, in what appeared to be a rural location.

At the family home, Dixon's wife Lynn answered her door at 8pm to two men dressed in what appeared to have been police clothing. She and Craig were persuaded to get into a waiting car, believing the men's bogus story that Dixon had been involved in an accident.