Shoe,
Thank you for that input. Spot on.
When I did my aeronautics degree (admitedly a long time ago) the AC is the point on a section or surface (a 3-D wing) about which dCm/dCl is zero, while the Neutral Point is the point on the whole aircraft about which dCm/dCl is zero.
i.e. the NP is the AC of the combination of wing plus tail.
So each surface has its own AC, and when I combine them into a whole aircraft I refer to the NP, and potion the CG relative to that.
Alasdair