New WP 3.8 install – permalinks bombing out to homepage

Hi, just installed a fresh 3.8 install, new subdomain, new DB, no plugins with a basic .htaccess in the root as per usual. Whenever i click on the permalinks option under settings it just bombs out to display the main page, which at the moment is the basic 2014 theme with the standard post. The only thing I can see different is that the link in the header bar adds a / after the ‘ptions-permalink.php’ – I’m assuming rather than displaying a 404 it is dropping to the homepage.

.htaccess is the standard one

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Anyone any ideas ?? Seems strange this has started happening …. I think its the last line of the .htaccess but if i comment this out i get 500 errors instead so there is something obviously not right ! PHP Memory is up at 256 and everything else looks fine ….