The source is from the war diary of the otherwise unremarkable 541 Searchlight Battery of the Royal Artillery, the commanding officer of whom had a certain knack for writing reports. The full passage was:
Heaven help any jerries who land anywhere near our Searchlight Sites. The Glasgow Scotties are tough. It has been suggested that Hitler dopes his Troops when they go into battle. A much better effect could be achieved in this Battery by dosing them with two pints of the wine of the County. It is aptly described by the Gunners as ‘Jungle Juice’.
While I was vaguely aware that the Germans drugged their soldiers, I believe with amphetamines, I was surprised that it seemed to be relatively common knowledge by this stage of the war. How and when was it discovered, and how did the news spread?
Followup Question, were the allies doing the same thing?
The answer to this is from what I can tell, not very exciting. It's that during the campaign in France, French and British forces took a comparatively small number of German soldiers as prisoners of war and discovered that they were equipped with "panzer chocolate" and "stuka pills", meaning amphetamines, specifically Pervitin. While we do not know the exact number of German POWs during the campaign in France, it is relatively secure that at least 400 German pilots and at least another 1000 German soldiers of various positions and branches of service were captured. Sine by that time, Pervitin use was already wide-spread within the German armed forces, this was easily discovered by the Allies, who also conducted interviews with the German POWs about that.
Nicolas Rasmussen in this article Medical Science and the Military: The Allies' Use of Amphetamine during World War II writes that:
1940 Germany’s stunningly successful western Blitzkrieg of 1940 spawned a host of rumors, including word of secret “Stuka Tablets” giving pilots of the dreaded dive-bombers a superhuman power to resist g-forces. Newspapers also contained reports of “heavily drugged, fearless, and berserk” German paratroopers. An investigation by the British War Office discovered that the Germans were using methamphetamine. Indeed, April through June 1940, the peak of the Blitz, corresponds to the wartime peak in the German military’s consumption of amphetamines. According to Steinkamp, the Wehrmacht used 35 million 3mg methamphetamine tablets for these three months alone, and much less thereafter. In both the United States and Britain, top-level scientific advisory bodies devoted to the military problems of “fatigue” earnestly began studying the effects of the drugs and their military utility.
For this he cites
Flying Personnel Research Committee minutes, record code PRO AIR 57/40 and 57/41, June 28, 1940, and November 9, 1940, Flying Personnel Research Committee records, United Kingdom national archives, Kew. Newspaper quoted in William Sargant, The Unquiet Mind: The Autobiography of a Physician in Psychological Medicine (Boston, 1967), 45–47
So while it is unclear which newspaper reported this phenomenon, it shows that both the Allied leadership were aware, most likely because of the POWs, and that this knowledge was spread within the general population through newspaper reports and thus can be easily assumed to have reached the lower ranks of the British army.
While Rasmussen goes on to relay how the Allies studied the phenomenon of fatigue and tried to address it from a pharmaceutical perspective, what I think needs to be taken further into account is that in terms of Armed Forces as an organization, they represent a true rumor mill. I have seen little serious study devoted to this topic when it specifically comes to the armed forces but especially the studies by Römer and Welzer/Neitzel about the protocols of Wehrmacht soldiers in Allied POWs – they study this in relation to the Wehrmacht knowledge about the Holocaust – show that knowledge about the enemy was fast to spread within armies and fast to become exaggerated – sometimes this even can be intended effect.
From my studies, there is the famous example from 1941 Yugoslavia where one German staff officer deliberately spread the rumor that the Serbian Partisans had cut off ears and noses of captured German soldiers. The rumor about the mutilation spread incredibly fast within the German armed forces in Serbia and resulted in a completely different unit committing a massacre among the civilian population in a village on the other side of the country. The officer who ordered said massacre directly referenced the rumor and it prompted the German commander of the armed forces in Serbia to issue a directive against the deliberate spreading of false rumors (not against massacring civilians though – that he still ordered).
Anyways, what this shows is that this kind of information, especially when reported in newspapers and discussed within the leadership of an army tends to spread fast. Since by July of 1941 the Allies were well aware of the German drug use, I find it not very surprising that the 541 Searchlight Battery of the Royal Artillery would know about it too.