We have not been able to identify this man with certainty. We know that he served with the Durham Light Infantry but there were 3 men called William Allen listed on the Commonwealth War Graves Commission website:

• William F Allen, service number 8585, died 21 September 1914, commemorated on the La Ferte-sous-Jouarre Memorial, Seine-et-Marne, France.
• William Allen, service number 21321, died 7 October 1916, commemorated on the Thiepval Memorial, Pier and Face 14A and 15C.
• William Allen, service number 30111, died 26 September 1918, commemorated on the Pozieres Memorial, Somme, Panel 68 -72.