Dear Stan, > thanks for a very useful reply! However, the link you mention does not > work. Is there a typo? Sorry, our website has just returned to its usual address: http://www.mrc-cbu.cam.ac.uk/Imaging/ and the temporary address that I gave: http://www.mrc-cbu.cam.ac.uk/Imaging/External no longer works. I'm very sorry, I'm afraid these things are beyond my control, Regards, Matthew %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%