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.
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Not everything works
I noticed that Blogrush and adverts on the site stopped working after I'd changed the Full HTML input format to include the HTML Corrector. There is an answer and fortunately it's quick and easy.