It is interesting to compare the tonal resources of the two organs of this thread. As regards tierce registrations, Manchester offers three options (Gt Cornet; Pos Cornet decomposé; Choir Sesquialtera) which - I believe - can be used on three separate manuals by means of a Choir on Solo transfer (not listed on the builder's website, but which I have seen on a photograph of the screen console). However, whilst such comparisons are practically relevant to some aspects of the performance of repertoire, a crucial consideration is that these two instruments have been built and are being rebuilt from fundamentally different starting points. Whereas Manchester is an entirely new instrument, reusing only a small selection of pipework from the previous instrument, broadly speaking, Canterbury seems to be the recreation of the spirit of the 1948 Willis rebuild.
Has the Cornet Voluntary offended? And are large swathes of the continental repertoire inaccessible? And will it sound incomplete? I suggest not. The performance of music always takes place within limitations; indeed, is this not the point of creativity which makes live performances so vital? Yes, whilst it will not be possible to register every piece of organ music, even large swathes of organ music, with stop names and combinations that appear and perhaps sound similar, there will nonetheless be many convincing and individual performances of music with the ample resources available.
The question of accessibility of repertoire is not just confined to stop lists. It is influenced by the voicing and scale of choruses, by the physical layout of the instrument, and its placement in the building. Even if this instrument were to have three separate tierce registrations, the difference between the layout of a werkprinzip instrument and the one proposed at Canterbury would already create a distinction, rendering, in the opinion of some I'm sure, an authentic performance already impossible. For instance, from experience, listening to some dialogical baroque fantasia in the nave of a cathedral such as Canterbury, some aspects of the music, notably the directness of sound and the spatial effect which were almost undoubtedly imagined and intended by the composer are completely lost owing to these factors. The situation is quite different in Manchester.
But OK, as a tierce rank is materially and financially relative insignificant in a scheme such as Canterbury, perhaps it would have been nice to squeeze one in somewhere. But then where does one stop? For me, the convincing performance of music begins and is most dependent on the technical facility and creative rhetoric of the performer. Yes, it's great to hear music performed on instruments that are as close as possible to those for which it was intended. But a critical aspect of the living tradition is the way in which music has subsequently been reinterpreted and performed in new contexts in a manner which is both respectful of, but not unnecessarily limited by the sources of that music.
As has already been noted, the prime function of this instrument is the accompaniment of the choral and congregational opus Dei, and all indications are that, post-rebuild, it will do better justice to this task with the expanded/restored tonal resources. The tonal palette offered is wide within the tradition of this instrument, and I look forward to hearing how organists will creatively perform Cornet Voluntaries and the aforementioned Messaien in time to come.