Jan may have hit this on the head; I missed the suggestion he cited. I have been ruminating about this; the "atrocity" seemed odd to me. To those who do not know him, "AOK4' is an exceptional, published scholar of the German Army of WW I.
The psychology of the event seemed all wrong to me. The Germans had just used a new weapon and easily drove the British out of their positions. I would think the prevailing mood among the Germans would be pleasure or even glee, not hate and bitterness. Some also may have been embarrassed or ashamed of having used a weapon that might have seemed unmanly. But not bitter hatred. I also agree with his assessment of the attitude of German officers; better that it was expressed by a non-German.
The prank hypothesis is probably what happened. The Germans, having skunked the Brits, were having some fun. Possibly they used the officer's uniform? I was also wondering how two or three men could wave a corpse that, with kit, might have even weighed 160 or 180 pounds on the ends of their rifles. They might have even broken their rifles; they are not made for such abuse. Even actually bayonetting someone might only involve a force of 10 to 25 pounds max., with dynamics, the stunt with a real corpse, with dynamic loads (pardon the mechanical engineer slipping out) could easily put a 100 pound load on the end of the rifle, easily breaking the fore-stock.
Mark, please be assured that Ralph, who I have had off-forum exchanges with, is a real gentleman and I am sure was not impunging your scholarly nature or instincts.
Until the prank explaination surfaced I was thinking that Corporal Holmes might have been approached by a psywar officer and asked to plant a story to tone up the home folks about the enemy. Certainly the British did a lot of "disinformation" during the war to, in particular, inflame American public opinion. But I have not heard about anything like that. Certainly after having been subjected to the largest flame attack of the war, to date, the British troops in the area must not have been in a good mood and would have been receptive to something like that. But I think that the prank idea leads the pack, and an actual atrocity having occurred is running in third place.