SLIPPED INTO AUSTRALIA without fanfare three years ago and unobtrusively aired by the ABC at 7.30 p.m. on Saturday, when A-list people are busy dressing for their glamorous nights out, the British television comedy series Doc Martin has become—boldly to borrow a phrase usually applied to movies about eviscerated, impaled and beheaded teenagers—a cult classic. To meet another Doc Martin cultist for the first time is to engage at once in a silent exchange of character and personality profiles. Some among the shabby, Saturday stayat- home elite have let the idea of having their own cult classic go to their…