You need 90%+ average across the A2 modules, and end up with 80%+ overall for an A*.
Maths is the exception, where only the compulsory A2 units included in the average. For maths this will be C3 and C4. For further maths, it is different again. As it is impossible for you to get an A level in further maths without doing at least 3 A2 units, your best 3 A2 units count. For example:
FP1 80%
FP2 100%
M1 100%
M2 75%
M3 90%
M4 80%
The best 3 A2 modules, FP2, M3, M4 average 90%+, and the overall 6 units average 80%+, so they will get an A*.