Adrian Riley (2007) seems to suggest that it is "Southern SSB" and is sub-specific cretaceus, Newlands (2006) says that those individuals from Portland are creataceus (possibly a spelling error?) wheras UK butterflies and the threads I've searched through say that cretaceus is extinct.
Reading that I'd made my mind up until I read the new edition of British Butterflies by Jeremy Thomas (1991, rev 2010) which agrees that cretaceus is extinct but then states that there are three subspecies of SSB, argus, caernensis,and the "blues of Portland".
So if I do visit Portland and find some SSB to shoot do I name them argus, cretaceus or SSB Portland variation, help!?

Wurzel