That attribute is deprecated. You need this: <ol style="list-style-type:upper-alpha;">
And please paste the URL of your blog when asking questions here, or link your username to your blog (as explained in the Sticky 8 Things to Know Before Posting). We usually need that to give a relevant answer.
Thanks so much for your help – will try that. The blog was still private when I asked the question, which is why I didn’t give the blog link, but I also didn’t realise it would be needed. It’s http://capclassique.wordpress.com