erm becouse its ilgal.
client confidentailety.
They can use their discretion and speak to third parties if they feel its in their clients best interest.
You might then say Ed Lawson telling Paul Harrison in 2002 that
"Bamber was a horrible man" and "
In hindsight he was guilty" was not in Jeremys best interest. But this author has a habit of recalling interesting conversations with people who just so happen to be dead and therefore are unable to confirm or deny if they took place.

Regardless. If we assume Ewen Smith did receive 11 diaries from 1969 to 1985 he still has five diaries unnaccounted for.
Depending on the situation in 1986 there would have to be a good reason why they were not used at trial in order to be an admissible gound of appeal.