The 42-year-old woman was approached from behind in an alleyway close to the railway bridge which links Grenfell Road to Kings Grove.
As she got to the middle of the path a man pushed her to the floor and demanded sex but a noise nearby spooked him and he ran off towards Boyn Valley Road.
The woman, who suffered minor injuries, ran to Grenfell Road and flagged down a passer-by for help.
The incident took place sometime between 11.45pm on Tuesday, September 6, and the early hours the next day.
Detective Sergeant John Slater said no arrests had been made despite enquiries and forensic and CCTV examination.
"I appreciate that the footage is not of the best of quality. However, it captures a man entering the alleyway where the offence took place, seconds after the victim did."
Contact Det Sgt Slater with information on 101 or Crimestoppers on 0800 555111.
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The victim of an assault in Cippenham was flown to hospital by air ambulance this afternoon.
A rapid response vehicle, an ambulance officer, an air ambulance and an ambulance crew were all called to an address in Earls Lane at 2.10pm.