Okay so I'm there, finally. A bit of trial and error got me there in the end. One of the main problems was due to browser caching, so I had to make some changes, clear my browser files down and restart the website to see the changes take affect.

1) As suggested, update the style.config file with your custom css file name like so..

<file>nivo.css</file>
<file cssvpath="/Data/style/nivoslider/default/default.css" imagebasevpath="/Data/style/nivoslider/default/">none</file>

2) Create your new css file with the following, and drop it in to skin folder.. same location as your style.config file (I found that 97% was perfect for me)..

div.gallerymodule { width: 97%; }
div.slider-wrapper { margin-left:auto; margin-right:auto; }

3) Clear your browser cache and start up your website.

These steps worked for me. Hopefully they will be of use to somebody else.