See
http://www.totnesonline.com/brunel/DCMS/
for DCMS & EH comments.
I am a bit surprised by the situation and EH stance that there two better examples of a very rare building type to justify not listing the Totness example. Rarity has been used as a criteria for listing elsewhere.
EH do make the point the building is altered.
The notion that because the system did not work they are less important is inconsistent with other buildings that are listed for example the building in Oxfordshire constructed with paper roofs are listed for that reason even though the paper was replaced many years ago.
The EH paper on listing selction for Transport buildings contains the following statement.
The 1840s saw a massive expansion in the network and while the Italianate style was favoured, many designs were eclectic. Great care should be taken in seeking out work of this date because, although much survives, it is often hidden by later alterations and extensions. Rigorous selection is required for buildings after c.1860: this reflects both the quantity of what remains and the standardisation of design. A number of factors should be taken into account when assessing buildings of the latter half of the nineteenth century.
Also of interest is this comment on the Starcross listing example example
"Other features associated with the pumping house, including a massive water tank, exist below ground and are of historic and archaeological interest. Starcross is the only complete Pumping House to survive from the 3 operational atmospheric lines in the British Isles."
Peter Wardle