Thanks Martin, Lyn and Mac for your further replies - you've given me much food for thought, and it's going to take a while to properly digest it all. Not overnight, but it will happen!
For now, I can say to Mac that the MIDI file in question was a standard BB SGU file - does BB produce Type 1 files? I opened another BB file at random to look some more at channel information in the event list window and found the bass on channels 11, 12, 13 and 14; the piano on 8 and 9; and the guitar all on 7.
Also, you mentioned "one patch per channel": seems quite sensible, but upon opening another BB file for a look, I found guitar on 12 through 16, and bass on 11 through 14 (assigned channels in the tracks window were 7 and 2 respectively, which I think is normal). So now we have patches sharing channels 12, 13 and 14 - it seems like the more I look, the murkier and more confusing it all gets. Is there any simple answer to all this apparent confusion, or possibly some further reading you could recommend?
Clinton.