Since the trial, it has also come to light that the hospital unit was suffering from a deadly infectious outbreak and had staffing issues which could have contributed to the rise in deaths between 2015 and 2016.
But infection was ruled out as a cause of death in each case, as I understand it.
And Letby was invited by the prosecution to state in each individual case whether it was her case that staffing levels, staff negligence, hygiene, etc contributed to that specific death, so where those factors were relevant, that's already been covered by her original defence.
I can't imagine she'll be given leave to appeal factors that they've already raised at trial. It will be like the appeal she's had, where the defence weren't allowed to call Dr Lee, the expert whose air embolism paper the prosecution had relied on, to testify on appeal, as they were asked why they didn't introduce his evidence into the original trial (in which they had months to do so) and they said that they didn't think of it until after they lost...
I doubt she's even got a strong case for incompetent counsel, as it was generally her own testimony on cross-examination that made her look more guilty.