The main reasons for Julie leaving Bamber on the 27th would be -
Couldn't handle the situation anymore - in her WS.
Constant jolly ups not for her.
Had a life & career in London.
Brett was always around.
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She kept in contact with him until the 7th September.
Once back in London she felt safe to tell other people which escalated to the police approach.
She obviously had to tell her friends in London Adam!
They would be asking about JB and she would have to tell them it was all over between them in a flood of tears!
Unfortunately things got out of hand,and to avoid feeling shame or inadequasy in front of them for being dumped, she exaggerated how evil JB had been towards her and eventually said that he had even killed his family!
From then on there was no going back!