The surprising thing is that it took the jury over 100 hours to reach their verdict, which suggests there must have been some doubters in there.
The great majority of the evidence was not direct but circumstantial. For example, the shift patterns which showed Letby present at every incident. Not direct proof but strong circumstantial evidence, which the jury had to carefully consider. Getting into the assessment of probabilities.
Babies were premature and under special care, so in these cases post mortems may not have been always carried out, as they should have been, according to guidelines – and that where postmortems did take place, they did not include systematic blood tests and toxicology. In some PMs air was seen present in X-rays, so we know now likely to be caused by air injection, but not definite proof which is why doctors let it go.
In one case, an extremely high insulin concentration (> 4,300 when 200-300 is normal) was found days after a blood sample was taken, but there was a very low C-peptide level (present when insulin is naturally formed) which meant the high insulin was the likely result of poisoning. But the baby had been transferred to another hospital after the sample was taken and it was assessed at the time by another doctor not in the neonatal unit, who did not suspect foul play and it was not reported back. This hard evidence was finally discovered two years afterwards by a neonatal unit Consultant carrying out a thorough review, instigated by the police investigation, of all the medical records of all the babies harmed.
The clearest lessons may be the dreadful management culture in denial/obstruction resulting in management preventing the police being called in. Also the lack of oversight over all incidents by people independent of management and medical staff .
Personally, I hope the parents sue the CEO and the Medical Director of the Hospital Trust for their alleged culpability in partly leading to at least the last two deaths and many injuries. Because the Inquiry will not be a statutory Judicial Inquiry these people can refuse to appear and cannot be held accountable.