Ok - have had someone technical look at this and got this email
" The problem is linked to mapped drives within windows, your colleague has a drive mapped for the server and the working folder to a letter i.e P(drive):
So when you try and open on the MAC, the mac is not aware of the drive letter so has to be relinked.
Possible way around this is to try and put the objects within the same folder and you use /<filename> "
I know packages like Adobe Indesign don't struggle with issues like this in terms of referencing files on a network - is it possible for SU and LO work in a similar way so that PC and Mac users can both reference files in the same way? Is this something that Google might address in the future?
Thanks