Right, here's the correct information - I've seen various truths and not-so-truths on here:
Three modules for each qualification.
A* in (normal) mathematics: Grade A overall (average) for AS & A2 units combined (six modules), plus an additional 90% ums average over C3 and C4.
A* in further mathematics: Grade A overall (average) for further mathematics AS & A2 units combined (six modules), plus an additional 90% ums average over ALL THREE A2 units (note not two like in normal mathematics).
The A2 units for further mathematics can be any EXCEPT the following: FP1, S1, D1, M1, C1-4.
The AS further mathematics units consist of: FP1 (compulsory), two applications (any, but, of course, not ones previously used in normal mathematics).
The A2 further mathematics units consist of: FP2 (compulsory), and EITHER two applications not on the forbidden A2 list above, OR FP3 plus one application not on the forbidden A2 list above.