A robber forced a 21-year-old man to withdraw cash from a machine in Market Deeping.
The victim was taken to the Barclays cash machine in High Street at about 9.30pm on Sunday, January 2 and forced to withdraw money.
Another 21-year-old came to his aid but the offender then assaulted both men near the Norwich and Peterborough Building Society. Police are keen to find witnesses and are particularly want to speak to a man and a women who were behind the offender and his victim as the cash was withdrawn.
Officers also want to hear from taxi drivers who may have seen the assault.