A couple with a small child have conned pensioners out of cash in two separate incidents.

The thefts occurred as Durham Police launched a campaign, backed by veteran comedian Ken Dodd, to encourage old people not to let strangers into their homes.

The comic's picture will feature on 270 bus posters with the message: Happiness is keeping your home safe and secure, don't let strangers in.

The force's head of community safety, Chief Insp Andy Reddick, said: "Ken Dodd is a very recognisable face and we think our target audience will relate to him. He is, of course, a very funny man but in this instance he is helping us get across a serious point."

But on Monday two incidents occurred inDurham that police believed may be linked to others on Tyneside and Wearside. A man in his twenties with a girl of about seven called at an elderly woman's home in the Springwell Avenue area, claiming to be collecting jam jars.

The child asked for a drink of water and she and the man also asked to use the toilet. They left, taking a shopping bag that included a purse containing £20.

Later a man and a woman in their twenties, again with a child, called at a ground-floor flat in The Chains, claiming to be charity collectors. The youngster asked for water but the occupants refused to let them in and kept them at the door where the child was given a drink.

The trio went to a flat upstairs and were allowed in and after distracting the two pensioners took a wallet containing cash.

Det Chief Insp Andy Summerbell said: "Our advice is when callers arrive unannounced and say they are from the water board, electricity board, council, or, in this case from a charity, they should be kept at the door until their credentials can be verified.''

The man is 25 to 30, 5ft 4in to 5ft 6in, slim and with thick curly hair. The woman is about the same age, 5ft 2in to 5ft 4in tall, with black hair and brown eyes. The girl is seven to nine, with black collar-length hair. Anyone with information should call police on (0191) 386 4929 or Crimestoppers on (0800) 555 111.