Create custom template of single.php for specific category

Hi

I am trying to set up a custom single.php file for a specific category. I am using the Thematic theme
I understood that Thematic theme would look for a slug relating to the post first.

The category slug is recent-projects . I have created the file single-recent-projects.php (copied the single.php content across) and have saved the file to the childtheme.

When viewing a single post from this category it is not using the single-recent-projects.php file. I am only wanting to remove the sidebar for this category post and leave the side bar for all the other category posts.

Am I supposed to activate the custom single file somehow?

I have been trying all sorts of things now for hours and none work. I can’t just comment the sidebar out in the main single.php file because I need it for other post categories.

Help please ??