Thanks for the help folks. The logical thing to do I suppose is to try it with the inbuilt mics in various positions, as per the above suggestions. I suspect though that we're going to end up using external mics and a mixer.

(Thinks. Here's me, a crusty old atheist, brought up in the best traditions of Marxist philosophy, and a champion of the musical outpourings of the lower classes. I never in all my born days thought I'd end trying to record a choir in a church.)