Routing
1. My knights reached the enemy camp and triggered the -20 BI mod which brought it below zero. The enemy army began to leave the field. For how long are they considered to be a routing army? ie how many rounds remain of the battle from that point? I assume that my player commanders continue to roll vs the UI of - probably - 1 but for how long?

2. Is BI tracked negatively? I know UI is always at least 1 but what about BI? If you assume zero to be the lowest point it can reach then decisive victories become much, much harder. But maybe that's intentional.

Routing
1. My knights reached the enemy camp and triggered the -20 BI mod which brought it below zero. The enemy army began to leave the field. For how long are they considered to be a routing army? ie how many rounds remain of the battle from that point? I assume that my player commanders continue to roll vs the UI of - probably - 1 but for how long?
See the results of

2. Is BI tracked negatively? I know UI is always at least 1 but what about BI? If you assume zero to be the lowest point it can reach then decisive victories become much, much harder. But maybe that's intentional.

Yes,it is.
Here is the improved list of Automatic events, waiting to appear in the Book of Battle 1.1.

Automatic Actions, Full
Certain Army Intensity levels are thresholds that trigger Automatic Events.
• When Army Intensity is 0 or less at the start of a second consecutive round, the entire enemy army routs. You have attained a Decisive Victory. Enemy unit maneuver is Run Away, and your unit may Chase.
• When Army Intensity is 0 or less at the start of a round, the entire enemy army hesitates. Heroes among them may attempt to Rally the Battalion (entirely at Gamemaster’s discretion). Enemy unit maneuver is Withdraw, Player units may Follow.
• When Army Intensity is 10 or less at the start of two consecutive rounds, the enemy army signals a general retreat at the start of the second round. Heroes among them might attempt the maneuver of Encourage Battalion (entirely at Gamemaster discretion). Enemy unit maneuver is Withdraw, Player units may Follow.
• Army Intensity from 11-29 has no automatic events
• When Army Intensity is 30 at the start of a round, your army has been pushed back and suffered significant losses. Your unit may attempt to Encourage the Battalion at this time.
• When Army Intensity is 30 at the start of two consecutive rounds, your army commander signals a general Retreat at the start of the second round. By command, all of your army’s units attempt to execute a Withdraw maneuver.
• When Army Intensity is 40 at the start of a round, your army is hesitating, wavering; and in the rear ranks the infantry are already moving away. In this round the maneuver of Rally the Battalion may be attempted.
• When Army Intensity is 40 at the start of a second consecutive round, your army routs. You have suffered a Decisive Defeat. Run.
In all cases, the First Charge (Round 1) Army Intensity counts towards the “two rounds in a row” to trigger an Automatic Event.

Thanks for the double quick replies, Greg. Just in time for my battle at the weekend.

I hope BoB 1.1 sees the light of day soon.