JK Rowling is a similar age to me and probably read similar books as a child. Not saying she has deliberately lifted anything but lots must have been in her subconscious mind.
I am shocked, after re-reading the Tim series recently, how it is shot through with Harry Potter similarities.
Carriages with winged horses that transport people to a Hogwart's like castle, these are invisible to some.
Tim looks similar to Harry and rides a broomstick.
Owls to take messages. I think this is so.
Tim's rather unloving Aunt floats to the ceiling and almost, rather mischievously, is going to float away until someone has a moment of conscience. She is a 'muggle' (although not by name) someone who does not know/see the magic.
Tim is a rather an unloved orphan, who lives alone in a small room.
There are other references in books that were around and read widely around the same time. One of the references was I think in Milly Molly Mandy, of all books, where a telephone box whizzes down through the ground.
Am sure there must be more, Worst Witch, etc.
Perhaps the skill in writing well/coming up with a new idea for children is to piece together lots of different sources to come up with an original tale?