Citation for VC reads: "The instance of devotion to duty by Boy (1st Class) John Travers Cornwell who was mortally wounded early in the action, but nevertheless remained standing alone at a most exposed post, quietly awaiting orders till the end of the action, with the gun's crew dead and wounded around him. He was under 16½ years old. I regret that he has since died, but I recommend his case for special recognition in justice to his memory and as an acknowledgement of the high example set by him."

(From wiki)
On 31 May 1916, Chester was scouting ahead of the 3rd Battlecruiser Squadron at the Battle of Jutland when the ship turned to investigate gunfire in the distance. At 17:30 hours, the Chester soon came under intense fire from four Kaiserliche Marine cruisers each her own size which had suddenly emerged out of the haze and increasing funnel smoke of the battlefield. The shielded 5.5-inch gun mounting where Cornwell was serving as a sight-setter was affected by at least four nearby hits. The Chester's gun mountings were open backed shields and did not reach the deck. Splinters were thus able to pass under them or enter the open back when shells exploded nearby or behind. All of the gun's crew were killed except Cornwell who, although severely wounded, managed to stand back up, and despite the entire gun crew around him dead or wounded, he remained standing at his post for more than 15 minutes until Chester retired from the action with only one main gun still working. Chester had received a total of 18 hits, but partial hull armour meant the interior of the ship suffered little serious damage and the ship was never in peril. The situation on deck, however, was dire. Many of the gun crews had lost lower limbs due to splinters passing under the gun shields. British ships report passing the Chester to cheers from limbless wounded gun crew laid out on her deck and smoking cigarettes, only to hear that the same crewmen had died a few hours later from blood loss and shock.

After the action, ship medics arrived on deck to find Cornwell the sole survivor at his gun, shards of steel penetrating his chest, looking at the gun sights and still waiting for orders. Being incapable of further action, Chester was ordered to the port of Immingham. There Cornwell was transferred to Grimsby General Hospital, although he was clearly dying. He died on the morning of 2 June 1916 before his mother could arrive at the hospital.