Hi Al,
there is no problem with crawling a site with mod_rewrite rules with sitemap generator. It acts exactly as SE bot does and just retrieves the URLs found on your pages.
I believe the problem is that you have the following tag:
<base href='
http://www.forces-of-nature.co.uk/store/'>Generator supports the <base> tag, but the attributes should be enclosed in double quotes ("). For the moment you can replace this line with:
<base href="
http://www.forces-of-nature.co.uk/store/">
The next version of generator will work with both quotes types.