Op I would like to advise you to take note of @Jggg advice:
"but also shift the blame to him and your family for not bringing him back sooner. They will use his poorly controlled diabetes not only to explain why he died, but to extrapolate to his character as someone who doesn't take care of themselves, and to explain that his life expectancy probably wasn't great either (even though no one has a crystal ball). It is not fair but that is how the legal system works and unless the trust thinks they don't stand a chance then it is going to litigation which you may lose (costing you money for solicitor) or win a sum of money that might not be worth it."
And to advise you to never ever again refer to his 'poor diabetic control again'. Don't give them that ammunition. You're going to have to fight hard.
They will fobb you off, somethjng chronic.
Does it say anywhere in his notes, by his diabetic consultant, diabetic nurse at Gp surgery, anyone else said once he got into hospital (in which case we'd have to question any basic nurse's - their medical authority to make such a comment?).
I've been diabetic my whole life, I monitor it closely, am on a cgm, and I can still go high, within an hour, and then low, even when I'm watching it and monitoring it every few minutes.
Focus on all their repeated negligence and their failures at every point, and try and play down, prevent them from focusing on your fathers supposed lack of care of his diabetes, (for which few people other than someone like me 
, is capable of commenting) although I'm sure they'll try!