Theme a specific custom post

Hi all

I site requires me to have a different sidebar appear on only a single post in a custom post type. I can create the sidebar, etc, and thought I would be able to do this simply by creating a page-id.php file and then including the specific sidebar.

However, when I do so, that template file does not override the single-posttype.php file. It works with static pages, not with single posts of a custom post type, I'm finding.

Is this correct? Can anyone help me by putting me right and/pr providing a workaround please?

Many thanks,