I’ve just been looking this up. As I understand it, the police mixed up the swipe-card logs by confusing exit- and entry-times to the unit. Entry was by swipe-card (to restrict access, for obvious reasons) while exit was just pushing a button to release the doors, no card needed.
The police mislabelled the two events so in the first trial, when evidence was presented in court that someone had entered, they had in reality left the ward, and vice versa. This was discovered and counsel were aware of it and had to keep reminding the jury to mentally reverse it. It ended up a total, confusing mess.
It also seems to have had a significant impact on the contention that LL was alone on the ward at various times because the error meant there were other staff there; the police were reading the logs wrongly.