There was an unusual feature (well for the LNER's London area, anyway) at New Southgate, which I think may just be visible in 2562's photo at the start of this thread.
In the Wallside siding, alongside and close to, the Down Slow, there was a wagon turnplate/table [possibly just visible over to the right in the photo] by which vehicles could be turned 90 degrees and passed across the public footpath and through the adjacent wall [archway definitely visible, by the footbridge] to/from the Friern Barnet Hospital (asylum)'s siding(s).
As wagons that were turned out of line with the main siding became foul of the Down Slow, the turnplate was unbolted by lever 1 in the signal box, which in turn would have locked the two home signals (& possibly the DF-DS points) which led to the Down Slow at this point.
Unfortunately I have no idea when this working ceased, the turnplate was secured out of use, or when it was removed, so this is of no help in dating the photo.
Finally, note the two tall telegraph poles, a feature of railway scenes then so normal as to be virtually unnoticeable, but now, according to observations/posts of recent months in the forum of website "The Signal Box"
http://www.signalbox.org/forum/index.php, almost eradicated from our rail network.
As a comparison, may I post here a scanned Kodachrome colour slide photo of my own (not a patch for quality compared with 'lifeboat1721's of Tornado at Carlisle I'm afraid) of a similar view, from around 1968 (see photo 'File comment' for main details).
Compared to 2562's photo, the public footbridge had gone, No.17 Up Fast 'Second Home' signal's co-acting somersault arms had been replaced by upper quadrants, the archway in the hospital wall was still prominent, and the Down Fast signal bracket in the background of the older shot, plus its Down Slow companion (not visible) had been replaced by a single BR(ER) 3-doll bracket signal (also virtually invisible, behind 4472's exhaust).
Regarding the maroon stock in the Wallside Siding, this might have arrived that day and just be awaiting its return working : I particularly recall an annual event of these times - some sort of National School Hockey final, played at Wembley, for which specials ran from the north to such as New Barnet and New Southgate, and from where the schoolgirl hoards were then taken round to Wembley by road coach. Southgate's train was then run-round, shunted across, then propelled back into 'The Wallside', and formed the later return working of the same trip.
But if I am confusing separate Saturdays, the set seen here may not be that one at all, but merely an out-stable from Bounds Green, possibly for several days.