Does anyone have any thoughts as to what the problem might be here?

The script uses the PHP built-in session mechanism so I think this is caused by the server settings (specially session.gc_maxlifetime). There are ways to overwrite the php server settings by the script (or by .htaccess file) but in case of the session settings this is a bit problematic.

Also out of interest I made a number of modifications to the BB-code function to enable users to include coloured text and to blockquote paragraphs, use a smaller font size for parts of the text, and a couple of other things.

The BB Code function is one of the changes in the current version (see this test posting).

Alex