In the last few minutes the CSS inserts in the settings seem to have stopped working. I don't think I changed any settings, I was just editing the CSS and suddenly it stopped working! The URL is http://www.shirleysa.org.uk.

What do you mean by stopped working? Did you or update any plugins? Had you made a change to CSS Inserts and clicked save when it "stopped working"? What version of Atahualpa are you using as I could not import the settings.

Atahualpa seems not to be reading anything in the CSS inserts box. I had made a change to the CSS when it stopped working, and as soon as it went wrong I reversed the change, but this had no effect.
However, I was just doing a bit of playing around, and discovered that if I add /?bfa_debug=1 on the end of the URL it renders perfectly. Confused! I am using Atahualpa 3.5.1.

Temporary solution: I have uncompressed the CSS and everything is back to normal. However, I would prefer compressed CSS to work!

Do you by any chance have a caching plugin active. If you do, shut it off when you are making design changes or they can take some time to show up since you may be seeing the cached page.

No, I don't have a caching plugin active. Whenever I make tweaks to layout, the changes usually show up straight away. I have enabled and disabled the compression a few times, and each time I enable it the problems resurface. Is it possible that there is a problem with the way Atahualpa is compressing the CSS?