[INM@RLM]

Larry, Thank you VERY much for rechecking this and giving us a clear source reference.
From what you found it is very clear that A.I.2(b) treated Feldflugplatz Barly as located near the Barly further to the east and nearer to Arras. Yes, the RAF may well have flown regular (quarterly?) PR coverages of that site to confirm there was no activity.

For the RAF this was only ever a "Former L.G." (RAF parlance), so there could be more than one way of credibly joining up the same set of dots.

If the original reference to the use of Barly as an airfield came from an Ultra text it would be a toss-up as to whether the RAF associated that with the correct Barly. PR coverage of an area where there never had been an airfield would consistently continue to show no signs of an airfield. Ditto intelligence from the French resistance. I'd doubt that a POW interrogation post-1940 would yield a precise a location: the POW would only know that there had been a Barly in use once upon a time. Unless he was well familiar with a very good set of French maps he wouldn't even be aware that there might be two Barlys set pretty close together.

Basing III./ZG 26 twenty km further east away from the other two Gruppen looks imo a lower probability than a closer grouping, whilst the possibility of the RAF associating the reference to a former temporary airfield with the wrong Barly looks about even stevens to me.
Undoubtedly the Luftwaffe knew which Barly they used. My suggestion is we keep an open mind for now, and wait for a Luftwaffe deployment map marked with Feldflugplatz Barly to surface - or an equivalent definitive contemporary Lw documentary reference with a precise location.