Adding a sub-site at "/blog"

I'd like to have a separate blog in my network installed at the /blog address but as I'm using sub-directory install it won't allow it, does anyone know of a way around this?

I've already modified the root site and removed all 'blog' permalink structures, categories, pages and the 'blog' value which is added to the 'Banned Names' box but none of this allows a new site with the name /blog to be created - I just get this:

"The following words are reserved for use by WordPress functions and cannot be used as blog names: page, comments, blog, files, feed"

Which is quite annoying :slight_frown: any ideas?