I discovered this photo on the English Heritage site “Britain from Above” (1) . It shows the site of the Royal Agricultural Show held in Reading in 1926. The area that became Henley Road Allotments is in the top left hand corner of the image.
There are other images on the show on the site but usefully this one has the same orientation as the plan in the Show Programme, below.
Comparing the two, the large tents in the image can be identified as the Flower Show. Next to these is the “Reserved Area”. This currently forms the centre of Reading Cemetery in Caversham.
There is a wider aerofilms shot too from a similar angle.
You can find the original Henley Road Allotments Post here:
(1) www.britainfromabove.org.uk. The digitised photos are from the years 1919-1953.