Puzzled by the very end, which seemed intended as a punchline, making clear what had really been going on, but for me as usual leaving me none the wiser, I found Katha Pollit’s contemporary review of this 1975 novel. This says a lot of interesting things, and the effect of reading it minutes after finishing the book itself means that I now remember, and think of, partly the novel through the lens of the review – and found nothing about the Whodunnit-style ending.
But now, after a full 10 minutes’ reflection, I suspect that the last sentence is a punchline, and the baby is Gerald’s after all, not William’s (I have no idea how long the action of the novel is supposed to have taken, but had imagined Gerald had left Britain too long ago for this to be possible – I now think I was wrong); it’s a new twist in a tale that has no ending, nothing neat.