Trevor -
I don't know if you are aware of the book 'Scenes Through the Battle Smoke' by the Rev. Arthur Male, but I was just researching through it (having read it a few years ago) when I came across the story you posted up about the Afghan in Sergeant Isaac Webster's tent..
"... With their lithe forms they [the Afghans] are accustomed to wriggle their tortuous course unseen along the ground, sheltered and concealed by every tiny bush or boulder. In this way, eluding most cunningly even the keen-eyed native sentries, they were known to creep into the very tents where the men were sleeping, to plunder, and even murder. A sergeant of the 51st, for instance, lying wakeful and restless in a tent under the very nose of a sentry, flung his hand careless over the side of his charpoy, and it alighted on the bare skull of one of these creeping robbers, who had got in and was lying there waiting for further opportunity. In a moment both men sprang up, and for a brief instant there was a life and death struggle. The Afghan, armed with his keen heavy knife, cut and slashed to free himself; while the sergeant, unarmed, and hampered with his heavy greatcoat, in which he was lying, could only hold on and shout for assistance. In rushed the guard, but out at the other side rushed the robber, breaking from his captor's grasp, whom he left behind bleeding from many a wound."
It's on page 76-77 (I have a c.1910 edition) - which you may already know.
Best -
Garen.