Jo,
I presume that you have started with what is available on the web.
Within Paddleducks, entering Compton Castle brings up ten existing messages (with your question as an 11th).
Google brought up ten references for Compton Castle, and about five for Cox & Co Falmouth, and those reference are likely to contain references to books, documents or other sources.
Given its long service, it may well feature in any book written about the local history of Totnes or Dartmouth or River Dart.
Regards,
Roderick B Smith
Rail News Victoria Editor