Limiting Posts Page to one category in TwentyEleven

So I’ve searched and searched and spent a few hours sticking code in various templates withing a child theme I’m doing for TwentyEleven, but to no avail.

From section on WordPress Codex about the Loop, it seems I should be putting this:

<?php query_posts($query_string . '&cat=-38'); ?>

just before

<?php while ( have_posts() ) : the_post(); ?>

But it doesn’t seem to have any effect no matter where I put it.

The set up is this:

Front page displays a static page:

Page called Welcome set as Front Page.
Page called The Blog set as the Posts Page (default template).

There’s a category I don’t want to appear on this Posts page. How do I make that happen?

I’ve set up a page showing just this category (id 38) which works, so I know the category number is correct.

Can someone please let me know which template I need to add the above code? I’ve tried quite a few!!

Alternatively, is there a way to do this via the functions.php in the child theme?

Any help much appreciated.

Cheers,

Tracy