Will have to re read that bit in Wilkes's book.
Apparently there was some ambiguity in blood expert John Hayward's evidence that there was nothing in the appearance of the silencer to suggest that the blood contained therein belonged to more than one person. Mr. Justice Drake took this to mean that the blood was unique to Sheila Caffell, and produced his own chart for the jurors to peruse with the five victims where Sheila's name was underlined. It's not specifically stated in Wilkes' book, but the implication is that since two jurors were already hanging out for not guilty it might well have persuaded a third so to do.
Of course, it would still have to mean that Sheila killed four, returned the silencer to the gun cupboard, then proceeded upstairs to shoot herself. I personally can't envisage such a scenario.
As far as Geoffrey Rivlin is concerned, it's stated in Blood Relations that though he couldn't be sure of Bamber's innocence he felt badly about the way the trial was handled, which included the summing up. It's the reason he spent many hours working
pro bono on the case after Bamber's legal aid was withdrwan.