Save WordPress RSS Feed to Flat File

This should be simple but WordPress is making my life difficulty. Essentially I want to output my WP's RSS2 feed to an external dir in a flat-file (e.g. mysite.co.uk/news/rss.xml). Normally I could do this with a simple mod_rewrite rule but WP doesn't seem to like this, the template system screws it up with "not a valid template" message.

So instead it'd be nice to have a simple PHP script that would save WP's RSS2 feed to a flat-file everytime a new post is added. This should be really easy to do but I don't quite have enough knowledge of WP to get one working myself.