Magazine theme drop down

Hi the drop down on the magazine them seem to pick up off the categories when i click on the list one on the drop down I get the post that relates to it but if I click a category above the final one it brings up a 404.

ie click st austell and a post appears but click cornwall and I get a 404 as mentioned.

I think its due to there isn't a post called cornwall but how do I get the drop down from the magazine nav to bring up all posts that mention the clicked on category.

the site is http://wordonthestreet.goodeye.org.uk

Can you check it out for me please.

Thanks

Dave