Police were called after a man was knocked unconscious outside the Dun Horse pub in Market Street Lane at around 4.20pm on April 24.
The victim – a man in his 30s – was taken to taken to hospital with serious facial injuries. He has since been discharged.
No arrests have been made at this stage.
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Officers on Monday (June 27), released CCTV images of four men they wanted to speak to as part of their investigation.
DC Paul Haworth, of East CID, said: “This completely unacceptable behaviour left a man with some very serious injuries.
“We have made numerous enquiries to identify the individuals involved and we now need to speak to the men in the CCTV stills.
“We know they are of quite poor quality but we hope that people may be able to recognise the men by their features or clothing.”
The first man is described as white, of heavy build, with dark hair and a dark beard.
He was wearing a grey hooded top and black trousers.
The second man is described as white, of heavy build, with light brown hair.
Police said he was wearing a red polo shirt and blue jeans.
The third man is described as white, of large build, with short ginger hair.
He was wearing a dark Adidas tracksuit.
“I would ask anybody with information on their identities to contact the police as soon as possible,” DC Paul Haworth added.
“I would also ask the men in the stills – if they see this appeal - to attend their nearest police station.”
Anyone with information should email [email protected] or call 101, quoting log number 1082 of April 24.