I persevered and finally got the midi to work with a BiaB style.

The problem I initially had was when I opened a midi file (using special), I was able to frame the chordshhet with part markers and all worked OK. This was expected of course because the midi was stored in the sequencer. I wanted to split the tracks out from the sequencer so that I could eventually bring in a BiaB style.
I had watched Henry Clarke do this by simply dragging the intitial midi file back on to BiaB and this was meant to automatically split the tracks over to the Untility tracks. It did do this but I discovered it was all out of sync with the original midi in the sequencer. I viewed the Utility tracks in the Tracks view and it was clear that tracks were not in the same position as the orginal.in the Melody track
After a lot of messing about I tried doing the job manually using the Track Copy/Move feature. A bit laborious but it worked.

I don't know if there is a bettter way to do it. It did work OK for Henry though