301 Redirect a Category

I am trying to currently alter my SEO URL's for the categories so that it is domain.com/category/sub-category.html and have tried

RewriteCond %{REQUEST_URI} /category/sub-category/

RewriteRule (.*) http://www.domain.co.uk/category/sub-category.html? [R=301,L]

However if I try accessing /category/sub-category/subcategory/ it redirects back to http://www.domain.co.uk/category/sub-category.html and I am unable to go deeper in the tree if this make any sense what so ever.

Thanks for any help