Lan (Saturday Night and Sunday Morning) Sillitoe, the young headmaster of England's new proletarian school of fiction, offers his seventh work in six years, a journal of his one month's stay in the Soviet Union. Alas, his prose is becoming as mixed as the critical reaction, both here and in his own country, to his increasingly assemblyine career. Here are tough, reather modishly intemperate sensibility, impressionistic hunks of travel writing heightened now and then by a keen poetic line, a brisk narrative style spiced with some brashly subjective opinions. Yet though the carry-over tance is a soi-disant revolutionary one- working class vs. the Establishment and so orth- in the end Sillitoe's salutations are really towards the extra-political: the Russians as people, the hearty hopefulness of the collective ""we,"" the glory of a land where God is deemed unnecessary- so different, he jejunely insists, from the bland boureois hypocrisies of home. Besides Volgograd (Stalingrad), the itinerary `includes Moscow, Leningrad, Siberia; a May Day parade feting Castro, conversations with the intellientsia, dinner with Siminov, visits to the Bratsk dam, department stores, theatres, ookstalls. The descriptions are generally excellent, and the book as a whole entertaining but ephemeral. Neither deep knowledge of nor any subtle response to the ommunist ideology and its socio-historical implications is ever manifested.