According to the Installation Blog for Archbishop Vincent Nichols, "Composer James MacMillan has been commissioned to write three enthronment antiphons for the Mass of Installation and Colin Mawby a trumpet fanfare. We hope to provide more information about this in the next day or so."
I don't have a fully-formed opinion yet of his music; you can hear some samples below. (I didn't really like any of them.) Has he composed any polyphony?
BBC Philharmonic - James MacMillan
Intermusica / Artists / James MacMillan / Biography
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VU-Kamerkoor: MacMillan - Te Deum (part 2 of 2)
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