A bit more info. I've now got it working - but only after I deleted the program from my Mac and then reinstalled it again.

I updated the basic program with the v.16 update this morning - I'm wondering whether that update had anything to do with it. As an aside, the hard drive now shows BIAB as version 16.0.16. After installing the updated program onto my Mac (using the BIAB Setup), the version now listed there is 16.0.14...