Alex
That's useful, thank you. Almost inevitably, after I'd posted, I found some of the answers to the questions I posed. Mode B is an aircraft identification mode (like Mode A). The difference is in the length of the gap between interrogation P1 and P3. (You'll recall that the SSR interrogator uses three pulses known as P1, P2 and P3. P1 and P3 are sent from the rotating array, while P2 is produced from an omnidirectional antenna. P2 is there to prevent transponders from replying to side lobes from the rotating antenna). Why they should choose to have two identification modes is beyond me. One source indicates that transponders may have Mode B as an optional extra.
The hunt for my other question - on Mode S - continues!
P