So I just realised that the rule book says you have to use the dice supplied in the core set - does that mean the official app is not tournament legal?

I believe the app is legal. Possibly you'll find that in the tourney FAQ..

Why would you want to use an app when you could be rolling dice?

The fantasy flight games star wars dice app is legal for tournament use. Your opponent will be required to double check the screen to makes sure your rolls are correct. Although the dice app tends to not roll as well as the dice (tends to roll average)

The fantasy flight games star wars dice app is legal for tournament use. Your opponent will be required to double check the screen to makes sure your rolls are correct. Although the dice app tends to not roll as well as the dice (tends to roll average)

This is very funny. Made my morning. Such understatement !!!