Hard to say without seeing anything. Is there any difference in the defined site vs the others? Is there a difference in the way the stylesheet is being attached? Can't see any reason for it to work on 2 other sites but not the third. I would start searching for differences between the sites.
We need to see the code.
Can you copy and paste it here from Code View?
Which code? From the template? Any specific lines of code?
Thanks for helping with this
Replies to several strands of the questions here ...
1. Here is the site as far as we've got http://www.sarah.uk.net/
2. It is not a purchased template. We started from a blank html page and then copied and pasted the CSS styles as they were created out of the template and into the CSS.
3. Here is a screenshot of the file tree
Does this help?