Thanks for your comments.
DiapDealer, I just tried the XML escaping. I notice the e-book viewer that comes with Calibre shows these as literal '<p>' characters in it's display, it doesn't actually use them for formatting. Don't know what others do. I see my Sony 650 doesn't seem to show the description field at all (or anything much except the title and author).
st_albert, it was largely because I thought that distributors (etc.) might use the fields that I wanted to try and get them right. However the more I look the more I think that distributors must not use them because they are rarely supplied with enough consistency (if at all) to make them useful.