A bag-snatcher who robbed an 82-year-old widow in a Hitchin alleyway on Monday was chased away by a passer-by – and detectives have today called for him to get in touch.

The witness, described as a grey-haired man, is said to have chased the offender away as he made off after stealing the bag in Elmside Walk – between Bedford Road and Fishponds Road, near Hitchin Swimming Centre – at about 3.20pm.

Local crime unit officer DC Kathryn Marshall – who is investigating – said today: “We need to identify the offender as quickly as possible and we hope that the man seen chasing the offender will be able to help us with our enquiries.”

She asked him to contact her on 101, quoting crime reference G1/17/3141, or to get in touch online at contacthertspolice.uk/report.

The bag contained a small wristwatch given to the victim by her late husband, which she had with her as it needed a new strap.

The daughter of the robbery victim – who wishes to remain anonymous – described it to the Comet on Tuesday as a ladies’ wristwatch with a broken pale blue leather strap, adding: “That can’t be replaced.”

The robber approached the victim from behind and suddenly tugged on her black quilted shopping bag, after which she fell backwards and hit her head – causing a gash that had to be glued in hospital. The attacker then fled without her seeing him.

The victim’s daughter said earlier this week that she was holding up well, but that the ordeal had left her mother feeling unable to walk down the alleyway she had used regularly for 55 years.

“It’s just heartbreaking that somebody could pick on someone as frail as an 82-year-old woman,” she said.

The victim’s daughter said the bag itself was of no great sentimental value, and that there wasn’t much money taken – but that she was very upset to have lost the watch, as well as her purse containing family pictures.

The robber has been described by police as a slightly-built black man with an unshaven face, about 5ft 8in to 5ft 9in tall. He was wearing a black baseball cap, a dark-coloured coat and dark trousers. He carried a black rucksack-type bag.

Anyone with information should call police on 101, quoting crime reference G1/17/3141, or Crimestoppers anonymously on 0800 555 111.