When you set WordPress up with:
Custom Permalink Structure: /blog/%postname%/
Category base: blog
Generated rewrite rules
WordPress generates these rewrite rules:
@jacob-peattie answered the question in his comment - I used the wrong key to set the number of posts my query generates. Swapping numberposts with posts_per_page solved the issue.
Here's the final ...
When I refresh the error page, it just refreshes.
If the rule has been put in the correct place in the .htaccess file (ie. near the top of the file before the existing WordPress code block) then you ...