A robber offered a couple a cigarette after threatening them with a knife and taking their mobile phone.
The man also stole £20 before he tried to make off on a bike but got stuck as his track-suit bottoms got caught up in the chain.
As he stopped to free himself he ripped his trousers and dropped the phone during the incident on Tuesday, June 16, between 6.30-6.45pm.
The couple, a man, aged 18, and woman, aged 19, were sat on a bench in the graveyard next to the High Street in Sutton Town Centre when he approached them.
The man later escaped on his bicycle with the phone and cash towards the cinema in St Nicholas Way.
A police spokesman said: “He was a lighter skinned black man, aged 17-18, about 5ft 8in tall, with ‘wonky’ front teeth.
“He was wearing a grey hooded top, light blue scarf and light blue tracksuit bottoms.
“He was riding a blue BMX bike with black handle bars.”
Witnesses should call Sutton CID on 020 8649 0747 or Crimestoppers anonymously on 0800 555 111.
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