DCI Noel McHugh, investigating, said: “Someone might see this video and remember that they witnessed this incident; you will see a female who is clearly shocked by what has happened and holds her hands up to her face. “You may have been part of the group and did not realise how seriously Desmond was hurt and that he has now died. That may prey on your mind. You can contact us and help us get justice. Desmond was an inoffensive man, a big man and character, who was out having a drink. The CCTV shows that he was senselessly attacked and he never stood a chance.

“The events of Desmond's attack precede the London Bridge Terrorists attacks which were in the following evening. So whilst there is a passage of time, people will remember where they were in London, and could remember this incident. I’m asking anyone who took any footage or selfies around that time in to contact us; take a look through your photos and see if you have something that can help us.”

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His sister, Vivienne Folan, said: "My brother Desmond was hard working and larger than life. He was on a night out in Trafalgar Square when he was brutally and viciously attacked by two cowards who then calmly walked away and left him for dead.

“As a direct result of this attack he suffered a traumatic brain injury and was left in a vegetative state. Desmond's life was ended far too soon and our family is left devastated by his loss.

“Unfortunately for Desmond, his attack happened less than 24 hours before the London Bridge/Borough Market terrorist attacks and I feel that possible witnesses may have focused their attentions on those events. I am appealing to members of the public who may have been in the Trafalgar Square area on the night and witnessed this assault to come forward to the police.

“I am trying to get justice for Desmond, his two attackers were very distinctive and part of a larger group. I hope that witnesses and those from that group who did not know how serious the attack was will now come forward."