I always asked specifically if there were points of accuracy on which they disagreed with Benn's representation in the published diaries. There rarely were, though several showed some disquiet at having what they thought were private political conversations reported in the public arena so soon after they had taken place.
I spent many weeks in the Benn archive over a four-year period and witnessed Ruth Winstone, the Benn diary editor, checking back to contemporary documents, including those on which Benn made his diary notes while attending meetings. There was also a second scrupulous check for accuracy when the paperback diaries were published and comments made on the original hardback were collated and, where necessary, the text altered to take account of them.
No diary gives more than the diarist's 'truth', but I am familiar with the other major texts and feel the Benn diary is as accurate as any political diary of the 20th century.