OH - hang on - it was because I was attempting to import head and neck into head only. I have solved this by exporting the head only, and it works, except that her eyes and upper/lowwer teeth are too high. I will try exporting them too, though I can't see any option to transform eyes on the Y axis, so I am not sure how they got where they are now?

 

EDIT: tried it, it didn't work, because I can't select the eyes, teeth etc along with the head before inporting the OBJ as a morph target (unless there is some way you  can tell me about?) So - I will export the morph targets for head, eyes, teeth seperately and see if that works...