Thanks very much Lawrence for the detailed explanation - appreciated. I was very much looking to understand the "mechanism" of how the protection worked. It's really fascinating to me how much goes into the design of these circuits - really clever and brilliant thinking.
And, yes, sorry I traced the S/BY to the the base of Q705, not the collector. I'll check if the collector to S/BY exists on the actual PCB but presumably it must based on what you've described.