First off I'd like to say that for some reason I believed that I had 2 different accounts. One logs in via Facebook - which uses email address 1 - and one that I log in to via username/password and has an entirely different email - email address 2.
I decided, I wanted to get premium and so, logged into my account on the desktop client via Facebook, I then went to process payment for the free trial. When it opened in the browser, I was logged into the other account using email address 2. I then went and added all my details to the account and got the 30 day free trial on the wrong account (email address 2).
Realising my mistake I removed the trial, logged out and applied my details to the account I wanted premium on - email address 1. When I did this it said the offer has already been used and is now going to charge me.