Access Denied on creating new site

Activated multisite, followed the instructions given but when I try to create a new site I end up getting an “Access Denied” message

Basically my installation is in a WP directory, rather than in my root public html

So when I first select add a new site I get page not found, but putting wp/ before the URL gets me to the link I want to create one

I enter the details, get the page not found error, so when I put wp/ in again, I get the access denied message

Any advice?