A man shoulder barged the member of staff at a branch in North End Road, before punching him several times, said police.
The male worker sustained facial injuries in the assault, which happened on Thursday, May 12, at around 2.35pm.
Police have released CCTV of a man they wish to speak to in connection with the attack.
The suspect is described as a black man, with a bald head, aged in his 30s and approximately 6ft1.
Anyone who recognises the man pictured or who has any information about the incident is asked to call Hammersmith and Fulham CID on 020 8246 2505. To remain anonymous, call Crimestoppers on 0800 555 111.