William is listed as being killed in action on 13 Jul 1917, the war diary for that day is transcribed below along with an obiturary from the Stratford Herald on 24 Aug 1917. It appears that William was killed during an enemy artillery barrage.
War Dairy
Lakehof Farm, Ypres 13th: Spoli Bank was heavily shelled from 6.30am to 9.30am with 5.9" and 8" but after that hostile artillery was quieter than usual for the rest of the day. The following man was awarded the Military Medal for Bravery in the Field and devotion to duty. No 41206 Driver H. Thirkell D/103rd Bde. END
Newspaper Obituary
KILLED IN ACTION
DRIVER WILLIAM COCKBILL
Mrs. Cockbill of Seggs Lane, Alcester has received the sad news of the death of her husband, Driver William Cockbill of the R.F.A. In a letter to Mrs. Cockbill the chaplain to the brigade expressed the deep sympathy of them all in the great loss she had sustained, adding that both officers and men spoke with great kindness of driver cocktail and with deep regret at his loss. They all felt that they had lost a friend.
It was something to feel that he had won the liking and respects of his companions and do any funeral took place at a small cemetery just a few miles behind the firing line practically every man in the battery was present to show the last signs of respect. Much sympathy is expressed with Mrs. Cockbill and her family in her bereavement.