Good luck with your Lloyd Carriers Alastair! And thanks for your educational input to this thread (along with everyone elses input, past and future).
I checked and a little more info on SG CI can be found at Dormer Tools website on pages 10 and 11 of
this PDF file. Basically it derives its desirable properties through the alloy composition rather than MCI's necessary heat treatment after casting. Although SG CI can also be heat treated if the resulting properties are required.
The 'white' and 'grey' CI's I've been involved with will all break up if you flog them with a hammer, but are in contrast with MCI and SG which seem to be - as the word says - 'malleable'.
Regards
Alex