You mean you want to be able to ARCHIVE it, not achieve it. You "achieve wisdom" as you get older. You "archive" files to be stored in another location.

If a topic dies and is out of date, I would like to be able to archieve it so it doesn't appear in the main forum, but could be readable if required. The way possibly to go is a another field which defaults to the main forum - ie the subject is seen, but is then selectable to archieve or off, so the subject is not seen, but is displayed in an archieve area of the site.

Anyone got any hints or ideas of how to do this?

I would create a category called "Archive".
If this category is only viewable by Admin / Mods you make it invisible to User.

Now you can change the Category number of any Posting which has to be archived.
I would do this by cronjob whith a little SQL query depending on Postings age;

If Posting time is older then now - 100 days... or what ever you want.

Regards, Willy