We know that Lewis was familiar enough to name one of his best books, The Great Divorce, in response to Blake's Marriage of Heaven and Hell. But I wonder if he had read the major poems.
""'When Aslan said you could never go back to Narnia, he meant the Narnia you were thinking of. But that was not the real Narnia. That had a beginning and an end. It was only a shadow or a copy of the real Narnia which has always been there and always will be there: just as our own world, England and all, is only a shadow or copy of something in Aslan's real world. You need not mourn over Narnia, Lucy. All of the old Narnia that mattered, all the dear creatures, have been drawn into the real Narnia through the Door. And of course it is different; as different as a real thing is from a shadow or as waking life is from a dream.'" (from The Last Battle, 1956)"