So I started the game for the first time and everything loaded fine, did the first bit in the dungeon and went to Foltest's camp, had the cutscene with the Ballista but after that I had to quit the game to look something up on the internet.

Then I restarted the game and now my save game won't load, it just goes onto a loading screen that appears to last forever :/

If you exited the game / alt-tabbed when the game itself is saving, that save file might in turn be corrupted.
You could try an earlier save or just start a new game.