Eighteen months ago, a two-man team clandestinely shot a film in South Africa and smuggled it to this country. The result, Last Grave at Dimbaza, was labelled a ' spy' film by the South African Embassy; but neverthless it aroused great interest at the National Film Theatre and at Cannes. This year it won the Committed Film Award at Grenoble. Tonight, for the first time in this country, its message reaches a wider audience; and in response to an invitation from Man Alive, the South African Embassy shows a film of its own in reply: Black Man Alive-The Facts. The two films represent remarkably different viewpoints on the conditions for black people in South Africa. In the last few weeks South Africa has felt the threats of expulsion from the UN and a new regime in neighbouring Mozambique. Prime Minister Vorster has promised dramatic changes in the next few months and Dr Kaunda has hailed a recent speech by Mr Vorster as ' the voice of reason for which Africa has been waiting.' Tonight Man