I have put a question by e-mail to Edexcel, and received a sensible reply!
Question:
"How can FP2 and FP3 be alternatives when the content of FP2 (for example hyperbolic functions) is a prerequisite for FP3?"
Reply:
"Regarding your query concerning FP2 and FP3, I would agree with you that they aren't really alternatives.
When the specifications were recently revised and C1-C4 introduced, QCA allowed us the option of a Further Mathematics combination which included FP1 and FP3 but not FP2; centres had been asking for this anyway when the units were called P4 and P6. It's down to centres what they want to teach and enter their candidates for.
A pre-requisite to FP3 says that a knowledge of hyperbolic functions is assumed; these would normally be covered in the study of FP2 but students who miss that unit would be expected to make up that section from FP2 separately."
Nice to get a clear and honest answer.
Aitch