. . . The Glasgow Highlanders' War Diary initially recorded that 'Capt [sic: by the time of his death his designation was Major] McCosh MC, OC "B" Company who was the only serving officer of the original battalion who had been continuously serving with the battalion was severely wounded. Capt. McCosh died at CCS.

Shoulder to Shoulder added a little more:
His death was felt as a personal loss by every officer and man of the battalion, in which he had served since 1914. He had been offered tempting staff appointments on several occasions, but he had always insisted on remaining with the battalion he loved. Mobilized as its most junior subaltern, he had commanded the battalion with distinction during the battle on Menin Road in 1917, and had acted for long periods as second in command. A Cambridge Rugby Blue, he took the keenest interest in all regimental games. Always cherry, always fearless, his popularity extended beyond the batallion through the brigade and division.

He is buried at Five Points Cemetery, Lechelle, a small cemetery now in the middle of peaceful fields. There are just ninety-nine other graves here, none of which are those of Glasgow Highlanders, and the casual visitor would not know, from what is on his headstone, that Maj. Edward McCosh MC who died on 26 September 1918 aged twenty-seven was a Glasgow Highlander because his unit inaccurately is given as Highland Light Infantry. . . .

. . . July 15th. At 5am orders were received that the Battalion was to attack the Switch Trench in front of Martinpuich. The Queens and Glasgow Highlanders were to carry out the attack. The Worcesters and 15/KRR (Kings Royal Rifle Corps) to be in support.
Our right was to start from the N.W. corner of High Wood (which we did not hold). The preliminary bombardment was to begin at 8.30am and the attack at 9am.
A message was sent to the Brigade that it was impossible to attack unless High Wood was first cleared. About 7.30am a message came from Brigade HQ that it had been requested that 7th Division should be asked to clear wood and asking if we had heard of any 7th Division Battalions were to do this. One of the C.O.s of a 7th Division Battalion said it had been suggested he should attack High Wood but it was too big a job for him.
About 7.30am a convoy of 16/KRR was sent up to be a support to the Battalion. This convoy & 3 platoons of No 3 Coy were put into wood to try & clear the Right flank of the Battalion when the attack began.
At 9am the Battalion attacked and only got about 150 yards coming on very heavy fire from wood and from Switch trench N of wood and from a sunken road in front of Switch trench.
The 2nd Worcesters were sent up in support but were also stopped.
The Battalion suffered the following casualties: 5 officers killed, 15 wounded and 1 missing.
Other Ranks: 87 killed, 214 wounded, 99 missing (believed killed).. . . .

Charles Reid was the officer in the missing list mentioned in the war diary.
His elder brother Charles Kirkwood Reid (later to become Colonel) wrote in his book "Shoulder to Shoulder" that Charles Douglas Reid had been killed while making "a gallant attempt to move his platoon forward" on the attack on High Wood. ("Shoulder to Shoulder" referenced by "Come on Highlanders!" by Alec Weir).
The War Office subsequently accepted that he had been killed on the 15th of July 1916.