When driving the car voltage is usually around 13 (on ford racing gauge), but when the fans kick in this momentarily drops to 11ish, then back up to just about 12.

However if its dark and I have lights on it is around 12 anyway and drops really low when the fans kick in, especially if I have stereo on as well. Having dipped beams and fogs on really seems to drop the voltage.

It may just be cossie paranoia but is this normal?

Does it sound like the alternator needs replacing? Battery has no problem holding charge, even if the car is not driven for a couple of weeks. Alternator belts were replaced by Reyland about a month ago.