My server host has now set the correct permissions and it’s resolved the CSS error message thanks.
“I’ve set the folder plugins/related-posts-pro/cache/ to 777, and the folder and contents of plugins/related-posts-pro/css/ 777, as indicated by the comments.”
However, I’m still not able to turn on the timthumb setting which is listed as highly recommended.
The plugin can be viewed here and I’ve turned on timthumb for a time so the error can be seen.