I think I may have answered my own question...
I started following a tutorial it took me ages to find. (won't let me link to it)
It said I should put this bit of code where I would ordinarily reference the stylesheet. e.g:
In my case it was:
<title>Library Info System</title>
<%= stylesheet_link_tag "style" %>
So I did this and then moved my stylesheet to /public/stylesheets/ instead of in the same directory as the layout file.
<%= stylesheet_link_tag "helloworld-layout.css" %>
Huzzah... It worked!
Sorry to have wasted anybody's time, but who knows, somebody else might find this useful.