Thanks Fernando.
I tried adding this and at first it repeated the description multiple times, so I changed the original block element from a Content Template to a Loop Template, and now I have managed to get the category description appearing once at the top of the page.
However, using the Loop Template, is there a way to set it so that the individual blog posts slot into each other in a masonry grid? I used to be able to set this when using the Customize settings, however now that I am using the Loop Template, this option is no longer available.
Thank you!