[Dave]

Just look at the URL structure and my previous rewriterule, you'll see that it's quite easy to make a rewriterule for that.

RewriteRule ^forum/(.*)/(.*)/([0-9]+)-(.*)$ showthread.php?$3-$4 [R=301,L]

However, this will not work to URL's that are constructed differently.
It will only work to URL's which are constructed like: /forum/section-name/subsection-name/threadid-threadtitle

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Also, you might want to include the following in your .htaccess file:

RewriteBase /

The absolute path is currently used on your site which causes 404 errors.
Or just include a backslash in front of the URL's in the RewriteRules.