well, I guess I may have been right about Long Way to Tipperary at least, this from a witness to the events:

(Probably) the most well-known Account Captain Sir Edward Hulse, Bart., 2nd Scots Guards

"...

On my return at 10.00 a.m. I was surprised to hear a hell of a din going on, and not a single man in my trenches; they were completely denuded (against my orders) and nothing lived. I head strains of "Tipperary" floating down the breeze, swiftly follwed by a tremendous burst of "Deutschland Uber Alles," and, as I got to my own Company HQ dugout, I saw, to my amazement, not only a crowd of about 150 British and Germans, at the halfway house which I had appointed opposite my lines, but six or seven such crowds, all the way down our lines, extending towards the 8th Division on our right. "