repmgr 3.0.4 and postgres 9.4 on ubuntu (ERROR: replication slot "repmgr_slot_2" does not exist)
OK, I found the issue. I hadn't properly configured passwordless ssh login between my master and slave. So despite being able to login into my master from my slave the issue was it couldn't copy files because ssh wold require a password.

My only issue is error reporting is horrible for repmgr.

Following this guide I missed a very small step. Always make sure you test the login of ssh between both master and slave for passwordless login to ensure it was setup correctly.