Just fixed it.
Turns out the BBpress SEO Tools plugin was stopping URLs from working, along with BBcode, post signatures and a few other little things.
Deactivating the plugin and using an alternative SEO plugin has allowed these things to work.
I’ve now installed the Zaerl editor which works, and the normal html tags are working for images and text effects.
My question isn’t as urgent anymore, but still any help on why the [ ] tags aren’t working will be appreciated 
