Custom post type not showing on main category archives

I am part way through setting up a new content type called "Walks". I am preparing to add various new custom taxonomies and custom fields, however I have just noticed that the initial category i have been assigning posts to is not aggregating the results.

To reproduce:

Got to http://handheld.simplecsr.com/walk/nettlebed-walk/
Click on "Countryside Walks" under the page title.
(This takes you to http://handheld.simplecsr.com/category/countryside-walks/ which display no results despite there being six walks listed in this category.

Can you please suggest the cause.

Many thanks
Chris