[ccp4bb] Rotation axis

[Phil Evans]

If I've go a superposition transformation (x' = Rx + t), as it happens  
from a superposition in ccp4mg, how do I get the position & direction  
of the rotation axis (to draw in a picture)?
I know that any (orthonormal) transformation can be represented as a  
rotation about an axis + a screw translation along that axis

I'm sure I've done this before ...
thanks
Phil