I'm now using Zotero to write an article and a few interesting points have arisen.
The JAVA uno error came up again when I tried to insert my first reference. I fixed this as before and citations have been working properly ever since.
You have to have Firefox open while inserting citations. This makes perfect sense since the citations are stored in the browser.
If you're unfortunate enough to have to cite using one of the many different Harvard styles, it's very difficult to see which one you should be using. For example, for my masters I need to use Harvard APA and for the article I'm writing I need to use Sage Harvard. These styles are not clearly listed in either the pre-installed list or the Zotero Style Repository. This means librarians may need to track down the correct house style or edit styles to get it right, ensure that their students know which one to install and use and that, for example, Harvard APA means the American Psychological Association style as opposed to the styles that actually have Harvard in their name.
Bookmarks and Reference Marks
Because I was working with OpenOffice, the default was to save references as ReferenceMarks. It seems that this format works only with OpenOffice - a bit pointless since most people I'm likely to send it to would be working with MS Word. In order to minimise the chances of the references disappearing, I had to save as .doc and select the Bookmarks option instead.
Other potential issues
I suspect that there could be a problem with the browser taking longer and longer to load if there are a lot of citations stored on it, but this problem has not yet arisen for me.
Once I sorted out these problems, I found it very easy to insert references and create a bibliography - it really is worth the effort.