For anyone who is also interested I have found the solution!
1. Create a folder "blog" in the root of you site, put the wordpress files in there
2. Import the existing WP database and set up, plus manage config variables in the WP database to point to yoursite.com/blog
3. Update the blog/.htaccess file to...
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
</IfModule>
# END WordPress
4. Update the SS3 .htaccess to
### SILVERSTRIPE START ###
<Files *.ss>
Order deny,allow
Deny from all
Allow from 127.0.0.1
</Files>
<Files web.config>
Order deny,allow
Deny from all
</Files>
ErrorDocument 404 /assets/error-404.html
ErrorDocument 500 /assets/error-500.html
<IfModule mod_alias.c>
RedirectMatch 403 /silverstripe-cache(/|$)
</IfModule>
<IfModule mod_rewrite.c>
SetEnv HTTP_MOD_REWRITE On
RewriteEngine On
RewriteBase '/'
RewriteCond %{REQUEST_URI} !^blog/(.*)$
RewriteCond %{REQUEST_URI} ^(.*)$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule .* framework/main.php?url=%1&%{QUERY_STRING} [L]
</IfModule>
### SILVERSTRIPE END ###
5. Theme your WP install with the same as your SS site
6. Use the RSS Reader widget to pull in latest blog posts from your WP blog install
Happy blog'in!