Can’t install to domain

I have WordPress set up and working at:

http://www.presagedesign.co.uk/wordpress

I have another domain at:

http://www.techietalk.co.uk

How do I get my techietalk domain to point to the wordpress install so it works?

I’ve tried setting the domains destination to http://www.presagedesign.co.uk/wordpress and while this works, it’s not right. All the links redirect you to http://www.presagedesign.co.uk and not http://www.techietalk.co.uk.

If I change the blog address URL in the settings to http://www.techietalk.co.uk then the site breaks and I get 500 internal server errors all the time.

I’ve tried following instructions here:

http://codex.wordpress.org/Giving_WordPress_Its_Own_Directory

But when I do this I get the scary error message:

Fatal error: main() [function.require]: Failed opening required 'http://www.presagedesign.co.uk/wordpress/wp-blog-header.php' (include_path='.:/usr/lib/php') in /homepages/41/d170503492/htdocs/techietalk/index.php on line 17

I’m probably missing something obvious, but I’m stumped. Can someone help?