This would all be fine except that I've been labouring for weeks under the impression that I would only have to play one service on Christmas Eve (the midnight communion service). In fact, there are two services on Christmas Eve, and I haven't prepared any music for the first one.
Now, most people would play the same organ pieces at both services, or look through their library for easy Christmas music from previous years to substitute. However, this is obviously cheating. Instead, I head off to the music library to look for Christmas music I don't have, and come out with the following:
La Mystere de Noel: Poeme Symphonique sous forme de Chorals Varies sur l'Hymne de Noel "Jesu Redemptor Omnium"
The composer is Auguste Fauchard, organist at the cathedral of Laval in France. It's reminiscent of Dupre's notorious Variations on a Noel, but easier to play and, as far as I can tell from my attempts at sight-reading it, possibly a better piece of music. To my knowledge, almost no-one plays the piece, although there is a recording by Fauchard's successor at Laval. Best of all, it would fit the service perfectly; with the theme and first few variations as a prelude and the final fugato and toccata as a postlude.
Of limited interest, I know, since you're not in a position to hear the music and can't judge it for yourself - but I'm very excited about making such a major repertoire discovery almost entirely by accident. Now I just have to learn it - only ten more practicing days until Christmas!