Ah, DS1 needs an A* in FM or a grade 1 in STEP for his 1st choice. 
Snowy, I think most schools have opted to keep the applied modules free, so haven't 'banked' the AS level, so they can be shifted around to get the best possible result overall, maths takes priority, then FM etc. Some modules are AS only though if taking maths and FM, I think, so can be shifted between maths and FM, but maybe not between AS and A2. 
If doing maths only I think you can have one AS module included in A2, eg S1 and D1 may be AS and M1 in A2.
DS's school has chosen not to do FP3 as a compulsory module. His FM will be FP2 and two optional modules. DS chose M2, M3 and S2, one will go with maths and 2 with FM.
This is all for OCR MEI, btw, so may differ for other boards, but I looked up the syllabus yesterday after getting confused myself between FP2 and FP3.