Don't root your blog at the root of your site and use a "noskin" page for your home page where you can set all kinds of filterings as desired.

Thanks. I've found the a_noskin.php and had a look.
I can't see any configurations to restrict the posts by category or add filters. I've had a search to look for examples but couldn't find much on this site or via Google.
Is there something I've missed? Can you direct me to where I can find more information on this?

Just add variables to force params in the PHP code. Look at this for an example:
http://b2evolution.net/man/glossary/generic-vocabulary/stub_file

Found the list of possible values that can live in the a_stub.php file. Please can you clarify the documentation for "cat" and "catsel%5B%5D" on the following page, as it's impossible to be certain what it means!
http://manual.b2evolution.net/URL_Parameters

Am I right in thinking that:

$cat = '-5';

will show articles not in category 5 and not in any of category 5's subcategories?

Yes this sounds absolutely right ;)

and I made a partial update of http://b2evolution.net/man/advanced-customization/technical-reference/url_parameters

In version 4.1.7 is appears to be broken :(

Entering http://www.bigsoft.co.uk/blog/index.php?cat=-5 forwards me to http://www.bigsoft.co.uk/blog/index.php/fun/ and only shows articles in the fun category (and below).

It appears to be broken in 5.0.5 as well. Can you check please.

I think the problem is in inc/collections/model/_category.funcs.php compile_cat_array()

I'd like to report this as a bug!

Is anyone able to confirm this?

Hi @davidnewcomb,

I confirm that something is going wrong when trying to exclude categories with minus ( - ). The development team will be reported about this. Thanks.