UKEng
Thanks for your input. Inevitably, shortly after posting I found some of the answers, including a schematic which gives the spacing between pulses for all modes.
So far as I can ascertain, it seems that Mode D was either experimental or simply not adopted internationally.
With regard to Mode S, accepting that it continuously sends a 24-bit unique address, I assume that (so far as you are aware) the Squawk code is there to show what ATSU is providing a service or what the aircraft is doing (ie general VFR on a 7000 squawk, aeros, para dropping etc) or have I understood the rationale behind the unique address, it's there simply for the interrogator "roll call" function?
So far as Blacksheep is concerned, you need a reality check. A quick internet search will tell you quite a lot more about transponders than I'm asking and, to my suprise, about the military equivalent (IFF).
If you'd like to be horrified some more, go here:
http://www.tpub.com/content/combat/14308/index.htm and then read chapter 8.
P