Nor was I saying that Wordpress was removing blank lines I had added, but saying that Wordpress was adding blank lines when I didn't want it to.
Also, if they listened carefully, they would also realise that I was not interested in paragraph spacing, but actual blank lines which it is adding.
Currently I have two blank lines between each paragraph, which looks really silly. I delete them, and they gradually appear again - and add more.
Edit: Just after writing this, I found the solution: https://gomakethings.com/remove-empty-paragraphs-added-by-wordpress/
Edit 2: Very glad I found this. It has added 15 lines between each paragraph!