The 1901 Census shows that Garstang had been born in Liverpool (on 11 January 1881) and, at the time, the family was living in Barnston in Cheshire. His father, also called Garstang, was then aged 43 and was a solicitor. His mother, Beatrice, was aged 37. He had older siblings - Vera (6) and Archibald (9). Garstang was three at the time. He is commemorated on the Memorial at Stockport Art Gallery, but it has not been possible to establish his connection with the area. He was educated at Uppingham School, where he was a member of the Officer Training Corps. His service papers record he was 5 feet, 7.5 inches tall and weighed 124pounds.
Garstang is believed to have been a member of the Cheshire Yeomanry, probably serving in England in the early part of the War. He is believed to have been commissioned as a 2nd Lieutenant with effect from 10 September 1916 and, a month later, is known to have been in Poona in India. The remaining records in his service file (held at the National Archives) are unclear as to whether he was then serving with the Yeomanry or a unit of the artillery. On 5 October 1916, whilst he was a patient at Sassoon Hospital at Poona, he was placed under arrest. This was for frequently remaining outside the hospital after its 8pm curfew, although he had been warned by the Medical Officer that he should not be out after this hour. He had been "censured" for the original offence and had continued to offend and was now "severely censured".
A year later, still in India and under medical care, he was recommended for six months leave in England on 27 November 1917. He embarked on 5 January 1918 and was, at that time, assigned to the artillery. Not long after arrival in England, Garstang received a letter at his home at 5 Inverness Gardens, Kensington, London informing him that an Army medical board considered him still unfit for service and in view that he had now had the maximum period of sick leave on full pay, he must relinquish his commission. It is possible, from the tone of the letter, that the army might be concluding that Garstang was not hurrying to recover.
However, the letter seems to have done the trick. Garstang appears to have then transferred from the artillery to the Cheshires and was promoted to Lieutenant in the April. However, he was once again in trouble with the military authorities. On 17 April, he faced a court martial for drunkenness and "conduct to the prejudice of good order and military discipline". He forfeited his seniority of rank and received a "severe reprimand".
On 27 September, the Cheshires attacked the enemy positions at Beaucamp, 20 kilometres south west of the French town of Cambrai. The Regimental History records "The village of Beaucamp was cleared by one company of the 1st Battalion, under Lieut. Lockett, after severe close quarter hand-to-hand fighting."
For this action, Garstang was promoted to Captain (Acting). He was also awarded the Military Cross. The citation published in the London Gazette reads "In the attack on Beaucamp on 27th September 1918, in command of a company, he mopped up the valley west of the village and part of the village itself, taking about 80 prisoners. Later he organized a section to deal with a machine gun post in the village and under his guidance they captured the gun and killed four of the enemy with it. His determined courage, energy and leadership were a fine example to all."
On 23 October, he led "B" company in another attack described here and was severely wounded, in three places. He was taken from the battlefield to either 29th or 46th Casualty Clearing Station, a few miles away at Beugny, where he would have received medical attention. Normally, casualties spent no more than a couple of days at a Station. They are either stabilised and moved to a more sophisticated hospital facility, or they have died. It must be presumed that either Garstang was too ill to be moved or that it was felt nothing further could be done for him, but he managed to hang on to life for several days more.