None of us can give an accurate reason why Julie stayed quiet - just a best guess. Personally I think even if Bamber hadn't dumped her, she would have eventually cracked one way or another. The point is though, that unlike some, she did come forward. If what she told the police was made up - where would that come from, there has to be a basis for such a lie. She couldn't know that the investigation was uncovering a different picture of who was actually responsible and as Jeremy's girlfriend, the relatives wouldn't risk recruiting her in case she told him. I personally don't think she told the whole truth, I think there is stuff she left out or which the police thought would be unhelpful in the doc. Julie isn't unique - I think she was immature and didn't realise the full impact of what she was mixed up in.