Thanks for all your help and good suggestions.
There
was a hole for a bolt under the oil pan but no bolt in it! Strange, the bolt's listed in the parts fiche but was mysterously absent on my engine!
Anyway, I removed the the shifter linkage and "Ta da" the cases practically fell apart. No need to remove the primary shaft, everythings in the bottom case.
So in summary it seems the correct proceedure for splitting CB550 cases is:
A) Remove every bolt you can find on the outside of the cases.
B) Remove single bolt under the oil pan (if present!).
C) Remove shifter linkage.
D) Strike firmly with a lump of wood and lift away upper case.
And then in my case; deal with the un-holy mess left by a broken con rod!!!

Thanks guys.