They have posted images of the documents on their web site. BTW, you misrepresent the case, because The Guardian is claiming only that when Botha showed interest in purchasing Jerico missiles Peres offered to sell them the complete system INCLUDING nuke warheads. But they pointed out that no sale went ahead because Botha baulked at the asking price.
Israeli PM to Palestinian President Abbas: We must fight terror together (Haaretz)
from the article: Israel denies offering nuclear weapons to Apartheid South Africa