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Etaples was a big military hospital near Le Touquet on the Northwest coast of France, but there are no records of admission, so we cannot identify when he arrived there. The only option is to understand the actions in which the battalion took part in the days leading up to this.
The Battalion diary for 1-15 April 1917 has been transcribed and can be seen by clicking on the button below. A map of the movements is also shown below.
This was the First Battle of the Scarpe, 9-14 April 1917, part of the Arras Offensive across a wide front from Vimy Ridge southwards.. The name comes from the River Scarpe which runs East to West just North of Arras and South of Saint-Laurent Blangy, Athies and Fampoux.
This web reference describes the battle overall. Wikipedia
Sidney marched from Arras at 2:30 am to get to their attack point ready for 5:30 am attack. They were in and around the two craters Claude and Clarence. On the inset image there is a small WW1 cemetery across the road from Claude crater.
The attack started with a barrage, but these positions were right on the front line and many shells fell short, killing and wounding several of the men around the craters. When the creeping barrage moved forward they moved into No Mans Land behind it and attacked the railway. At midday they attacked the Point de Jour, a German stronghold, with the 11th Battn Royal Scots.
They consolidated at the Railway that night and 10th & 11th, then moved forward via Athies to the trenches at Fampoux. Over the course of the month they made significant gains in territory pushing the Germans back.
From this Sidney would have most likely been wounded on 9/4/17, either because of our barrage falling short into the craters at the start of the day, or in the subsequent fighting during the day. There are no reports of casualties after 10/4/17.
Overall, the Third Army had 87,000 casualties across the Battle of the Scarpe, which was a large force. This includes wounded as well as killed, so there would have been many wounded, and potentially Etaples hospital would had many of these.
This link give more details of the battle field Battlefield