Wish I'd have found the HTML Corrector module for Drupal earlier. I've noticed some odd behaviour in a few places and so I'd restricted my articles to being mainly text-only.
Looks like the issue with the sidebar at the bottom was possibly created by the teasers not having all the html tags closed. This is something that I, as a writer, have no control of. Seems quite a basic error in coding, especially in parsing and generation. Fortunately though, it looks to be becoming part of Drupal 6 when it arrives.