To help future people searching for Evernote appending.
Lesson 1: Have a note in place ready to accept the incoming messages for appending.
I did not have a note already in place to append to, so my trigger created a new note. This is not a problem except that I had three triggers within seconds of each other. When my second and third triggers fired, the Note had not yet been created, therefore Evernote created new notes for these too.
A few days on, my triggers are now happily appending to my single Evernote.
Lesson 2: Having liaised with another service provider, I am a bit more aware of the role of Evernote.
My understanding - Evernote provide (and support) the specific services available to IFTTT - currently two triggers (New note in a notebook, Add a specific tag to note) and six actions (Create a note, Append to note, Append a to-do note, Create a link note, Create image note from URL, Create an audio note from URL). IFTTT are responsible for implementing the services correctly when they create their interface to the service. Unfortunately, it is not always easy to establish which side a problem may be.