I wanted a time, and I wanted a location. That would seem to be a reasonable evidence base. Could the OH story, the most likely candidate, be authenticated? If it really was centred at "the southern point of la Boisselle", then the unhappy candidate would be in the brigade opposite, the Tyneside Scottish.
The brilliant John Ferris led me to a document written between Charteris (Haig's Chief Intel officer) and MacDonogh (Head of GHQ intelligence): WO 158/897, dated 5 July 1916. This was dated for the fall of la Boisselle, but can't of course pinpoint that this was the particular gadget in question, tempting though that would be to think so. It includes the lines:
"Among other things in the loot was a secret box and showed beyond all doubt the Germans are making use of their apparatus for overhearing out telephone messages and are getting news of practically all our intentions...There is no doubt that by the telephone the Germans were able to avoid a tactical surprise north of the Somme. South of the Somme it was a surprise both tactical and strategical."
So at least we had a major security problem associated with the Somme attack in a documentary source, not a piece of paper possibly found in a dugout by an idle soldier some months later.
I'd been in the archive looking at the 179th Tunnelling Company for all the camouflet material needed for the 9th July episode. So I was delighted to find the following in their war diary: a report undated but about their investigation of the la Boisselle mining system upon capture, so roughly around that first week in July. It goes on to say, under the heading 'Moritz System', a reference to the German listening device, "In going through his listening room at I.8.2/3 I found certain papers that made me suspicious. They were obviously copies of British signal messages recorded by a German from the style and characteristics of the handwriting, spelling etc."
I felt we now had a document containing British orders in a German position opposite the Tyneside Scottish. But had they been used to alert the Germans?
Further conversations with Jack led me to his summary of the memoirs of Generalleutnant Freiherr von Soden, CO of the 26th Reserve Division, in the line opposite the Tynesiders.
Referring to some parts of the Somme attack coming as a surprise, he goes on to say "...the Moritz station near Contalmaison made up for the deficiency in the early hours of the 1st July when it intercepted an order from 34 Div, which made it quite clear the offensive was about to begin."
Von Soden published his memoirs in 1939, and one wonders if Miles' OH drew on this book while it was being prepared for publication.
That seemed a good solid base of two original contemporary sources and two official ones, so I decided to go ahead with the story.
Why is this not a bigger part of the national remembrance of the Somme?
I think it has just been too awful, too human, too dreadfully ordinary for us to contemplate as part of a story of such loss.
As the drama of record of the signallers of WW1, TOMMIES could not shy away - understandable though that might have been - from telling what might prove to be an unpopular story.
2019 update - subsequent conversations with Intel historian Jim Beach make me happy with the above reasoning, indeed, he even quoted it in an academic article - I'll get the details.