Callum, i recently got to that stage, and i think the best way to learn backflips is to join a trampolining class. lol...
I've never tried one yet, but one of the people at the gymnastics i go to also runs a trampolining class, so i'm going to join that and start working on more advanced moves.
I suggest you do something similar if you can
Or at least get someone who knows how to do them to show you how to do it first hand...