We finally finished it yesterday.
As you can see, J plays while we read. Sometimes he would bang about with his toys and makes spaceship sound-effects and not seem to be paying attention, whereupon we would halt with a huff and threaten to stop reading. But then time after time, when K & I were scratching our heads trying to recall an earlier episode in this long story, J would cooly note exactly when it was and pretty much exactly what was said. D'oh!