I assume the book you are referring to is Dirty Politics. If so it cant be used for anything as all the 'evidence' in it was obtained illegally, unverified as to its authenticity, cherry-picked, taken out of context, and published without giving any person it was aimed at a right of reply.
"…they knew they had accessed the data without Authorisation". Please ask this commenter how anyone can be deemed to have accessed data without authorisation if authorisation was not required or needed to access it?
"I hope they are prosecuted for it". Please ask this commenter what they could possibly be prosecuted for? I can't think of anything as a crime was not committed.
"At what point did Slater tell Labour about their security breach?". What security breach? There was no security to be breached. None.
When it came to Slater informing Labour about their data having zero security. He told Labour and everyone else about it at the same time. He even put it on youtube. And after that, unbelievably, the powers-that-be within Labour still didn't adequately address the issue.