Popped back over there this afternoon. Your little book on Essex could be right; looks like there are two small puddingstones in the tower above the lower window on the west side (though they're a bit too high up to be sure) and another possible on the north side of the tower. Got talking to someone who lives across the lane from the church and who had some interesting info on the springs, streams and Roman remains in the immediate area - will add them to the field notes eventually.