There is a photo on the wall. It was taken, most probably, in the spring of 1915, and shows eight uniformed men in the jaunty confidence of youth, bedrolls slung over their shoulders. They stand, arms around each other's shoulders, caps askew, one with a cigarette in his mouth, another with a pipe. They smile cheerily.

The bright spring sunshine leaves deep shadows on their foreheads. In the middle, arms folded, a young man with a heavy moustache leans on a road sign: "DANGEROUS! KEEP OFF THE TAR". This is my great uncle Charlie. He has a Red Cross badge on each shoulder and grins broadly.

In his entire military career Uncle Charlie won no medals for bravery, never advanced beyond the most junior rank in the army and almost certainly neither killed nor wounded a single German. He had enlisted in the Royal Army Medical Corps and his job was to save lives, not to take them. The 1911 census records Charles Edmund Dickson as a twenty-year-old living in Shipley, working as a "weaving overlooker" in one of west Yorkshire's numerous textile factories.

Uncle Charlie looks a slightly unconvincing soldier, cutting none of the elegant dash of glamorous young officers like Rupert Brooke. He fills the uniform, for sure. In fact, he looks as if, with a bit of time, he could more than fill it.

On 7 August 1915 this cheery young Yorkshireman, with his affable, cheery face, was killed in Turkey. His detachment of the Royal Army Medical Corps had been despatched to Gallipoli as part of an ill-conceived attack on the "soft underbelly" of the enemy, its purpose being to relieve the stagnation of trench warfare in France and offer a decisive breakthrough.

Uncle Charlie was my mother's father's younger brother, dead well before she was born. Yet as children we were all familiar with him seventy or more years later - Uncle Charlie was a present absence. My mother made the pilgrimage to seek out Uncle Charlie's name among the thousands etched into the wall of the Helles memorial. Someone helped her find it, and it turned out to be so high above her head that when she posed for a photograph she could only point it out with the aid of a branch cut from a nearby tree.

Family legend had it that Charlie had faked his age when he signed up and that he was cut down by machine gun fire as he waded ashore on his eighteenth birthday. This was plainly untrue - his twenty-fourth birthday had occurred almost six months before he was killed. But this imagined version of his death seems somehow to express a greater truth than the mere facts. Can there be a family in Britain which does not have some similar ancestral story?

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