Can I export a BIAB file in to Sibelius to read just as a LEAD sheet? I create lead sheets in BIAB because then all my work creating them results in a backing track I can rehearse to but BIAB has some frustrating limitations to lead sheet notation control so after creating a BIAB file I wish to export the info to Sibelius so I don't then have to create a whole new lead sheet in Sibelius as well. Is this possible? I just tried exporting midi and of course it looks like a full score not a lead sheet.

Save Special > Song As MusicXML file > Sibelius or MuseScore > Open

OR Save Songs As > SGU > MuseScore > Open > Export > MusicXML > Sibelius > Open

You may have to do some editing and deleting but MusicXML is your best bet. For a lead sheet, export only those tracks. Here's the page from the manual.

Saving as MusicXML

OK thank you i'll follow those instructions and give it a go

Hi Mike is that advice just pertaining to Windows or a later Mac version? I'm on Mac 2014 and there is neither option for saving.

The OR option is for earlier versions:

OR Save Songs As > SGU > MuseScore > Open > Export > MusicXML > Sibelius > Open

Yep, BIAB didn't support MusicXML till 2016. It's a good idea to list your exact version in your signature so that responders have the info they need.

musescore has a limited ability to open BIAB files saved as .sgu. Unlike MIDI (.mgu), this will be backing tracks and chords only. It may do what you want or not. Fortunately, it's free.
musescore
Otherwise, you're back to MIDI and editing from there.

If that's not acceptable, probably time to consider an upgrade. There's a new, 64 bit version for Mac on its way but only PG Music knows when it will be here other than sometime in 2019. Rumors are that the beta version has yet to be released so it won't be really soon.