Woah, so much misinformation in this thread!
The criteria for A* are as follows:
80% average across the entire A-level.
90% average across A2.
You don't need to get above 80/90% in each module, just on average. And providing you've achieved 70% in your AS, you won't need to worry about it in the slightest (as far as the A* goes). If you get 90% in your A2 - the minimum required for an A* - you will automatically achieve the 80% across the entire A-level.