Here are a collection of close-up images of the top left corner of the stamps from varied sources. They were taken at the same time, same  position, same lighting and same camera settings. Apart from resizing for comparability there has been no manipulation of the images. It seems that the use of the camera accentuates the colour differences between  the stamps, but these images can be used to compare the resolution and  high detail print patterns of the different stamps. Click on any image for a larger version.

                     Pre-Bath test print                      from a Wincanton perf                      from an Enschede sheet

             First Pseudopolis cover                       Second Pseudopolis cover                  Third Pseudopolis cover

Some observations are immediately apparent.
The Pre-Bath print is a lower resolution than the Wincanton perfed stamp. It is known that during their lifetime attempts were made to improve  quality, so the Wincanton stamp may be from a later batch. I need a Bath perforated stamp in order to draw further conclusions regarding the earlier prints.
The Enschede stamp and that from one first Pseudopolis cover are clearly the same print. The slight colour  difference will be due to the white envelope as a background. These are so crisp you can see the detail of the cabbage hearts (just).
The  stamp from the second Pseudopolis cover bears a great similarity to the Wincanton, but it is not quite as good a print, and is definitely a paler print. Again the background (a brown envelope) may have affected the colour the Pseudopolis cover stamp, but by eye there is a slight  difference in hue between the two stamps.
The stamp from the third Pseudopolis cover has the lowest resolution of all, and is clearly from a different printing. The apple-green colour does reflect the colour shade of the stamp. This stamp has perhaps the highest contrast between  light and dark of all the printings; the background the the value text  is particularly intense.

From this and the other observations I feel I can compile the DWS Completist’s Checklist of stamps.

Stamp source Perforations Sheets of Overprint Other 1 Pre-Bath test print imperf 4     2 Bath rat nibbled 12/2 rough 28     3  Bath bees waxed 12/2 28     4 Wincanton early 10/2 28   lower definition 5 Wincanton later 10/2 28   higher definition 6 Enschede 13.5/2 20   larger stamp design image, smaller borders highest definition prints 7 Going Postal 10.5/2 88   inkjet print with dragline flaws smaller measuring 35 x 30 mm 8 Clerk of Works refund 10/2  28? ¼p addition small ¼p 9 Clerk of Works refund 10/2  28? ¼p addition larger ¼p 10 1st Pseudopolis cover 13.5/2  20 hand-written 3 Enschede print 11 1st Pseudopolis cover 10/2  20 hand-written 3 digital print paler, more emerald print 12 2nd Pseudopolis cover 10/2  ?   digital print 13 3rd Pseudopolis cover 10/2  ? 8 pence digital print larger stamp glossy surface high contrast image apple green