He sings as he strolls jauntily through an alleyway in World Square, holding a cigarette and a plastic bag containing what police think could be a six pack of beers.

He looks like a thousand other young Sydney men; his jeans slung low on his hips, an open flannel shirt, tight white singlet, matching white trainers and his hair cropped in the spiked fashion favoured by flashy young NRL players.

The youth police believe fatally attacked Brian Gilsenan, right, for no reason.

Except that within an hour and a half of being captured on this closed circuit television footage, this young man and his friend had bashed a father-of-two within an inch of his life in what police say was a random attack.

Their victim, 54-year-old Brian Gilsenan, had been at a work function in Darling Harbour and was on his way home to Chatswood when, for no apparent reason, they went for him outside an apartment block on Bathurst Street just after midnight on February 8, 2008.