Experienced Wordpress developer needed to create custom walker for menu.

I'm working with a purchased theme for a Wordpress website - http://themeforest.net/item/base-premium-knowledge-base-wiki-faq-theme/4636112. When I have Primary Navigation set to use my custom menu (Main Navigation), naturally it uses that structure.

If I leave the Primary Navigation drop down blank, no menu is displayed in the header, whereas in the footer, if no custom menu is selected from the drop down, it defaults to show whatever pages are set at the highest level, even though they're not part of any menu.

This is actually the ideal scenario for me - I'd like the header and drop in menus to do the same thing. In updating this website, I will be adding lots of pages, and I would prefer it if I didn't have to add pages to the menu manually (achievable with the 'automatically add new pages' checkbox, but if I were to then change a page's Parent, I'd need to change it in the custom menu too - not ideal).

To explain in another way, I want those menus to include all pages from WordPress - not just the ones that are added to a custom menu.

The original code used the wp_nav_menu function. I've changed that to the wp_list_pages which has successfully listed the pages, but because the HTML markup is different, the CSS isn't carried over and it displays badly.

I'm aware that by using wp_list_pages(), I need to use a custom walker. e.g. http://badfunproductions.com/create-a-custom-walker-class-to-extend-wp_list_pages/

Here's a link to the site: http://raw.your-hub.co.uk/ If you highlight the header, you can see how the menu is currently displaying in the header (the text is the same colour as the background, so it can't be seen without highlighting).

This should be a fairly quick and straightforward job for someone with the right knowledge. If you're not familiar with Wordpress walkers, or don't know how to do this, please don't submit a proposal. Thanks a lot.