Thanks for the reply!

I don't think it's a style issue as even when I try to change a certain paragraph into a certain type of style the others persist and do exactly the same thing.

It seems any document I try to format in my copy of word does exactly the same thing as this screenshot shows. http://i.imgur.com/tnbgh.png
The highlighted text is the part I attempted to double space, but as you can see the rest of the document also became double-spaced.

I've tried starting new documents in Libre Office and they do exactly the same thing so I am starting to wonder if this is a computer issue rather than an issue with Word.