Where do you think it might be? Westminster Cathedral? Spanish Place? Farm St? London Oratory? Church of the English Martyrs, Cambridge? St Mary's Cathedral, Sydney? Impossible - they all celebrate Ascension on Sunday (surely the policy of moving important feasts and solemnities to the nearest/following Sunday will one day come to an end?)
Whatever the venue, to sing Credo I you would have needed a book with a Kyriale inside (e.g. Graduale Romanum, Singing the Mass). To sing the Propers, the choir would have needed the Graduale. You would have heard them sing the exquisitely beautiful introit Viri Galilaei.
The other parts of the Ordinary were in Greek/Latin - from the delightful Rheinberger Cantus Missae.
It simply must have been in a Catholic church - any Catholic church, since this is the music that has pride of place in the Roman tradition, that constitutes a treasure of inestimable value.
If you had been present in Cambridge on Thursday 9 May 2013, it would have been all but your bounden duty (after having punted on the Backs) to attend this sung service, and you would have heard Catholic music given the pride of place that it so deserves.
You would have heard the Church's own music for Ascension, sung on Ascension (how novel).
You would have been at ... King's College.
Kudos to Stephen Cleobury and the staff of the college and chapel.