I finally fixed this minor problem! Only took a year or so!
AddType text/css .css
This optimiser complained.. (although it was because I got the css url wrong but the result in the end is the same!)
The cookie settings on this website are set to "allow cookies" to give you the best browsing experience possible. If you continue to use this website without changing your cookie settings or you click "Accept" below then you are consenting to this.