Display specific category template for posts

Hi all

A very simple question – I have three main categories on my blog – with some subcategories each.

I have created 3 category templates, thus – 1 for each main category.

What I want to achieve is
1) when a sub-category page is displayed, the template for it’s main-category is used
2) when a specific post is displayed, the template for it’s main category is displayed

So – if I have “main-cat 1” with “sub-cat-1a” – when displaying the “sub-cat-1a” page, it should actually use the “main-cat 1” template.

And when displaying a post that is in “sub-cat-1a” or one that is in “main-cat 1”, again it should be displayed using the “main-cat 1” template.

How do I achieve this?

Thanks!

thomas