Thinking of you, Kate, and echoing what Nell just said. your dad is surrounded by love, he knows you're there, and should he decide to go while you've gone out to make a cup of tea or snatch half an hour's sleep, he will do so knowing you and your mum have been there all the time. You can't do any more than you are doing at the moment, but I know it's hard.
I was with my gramps and uncle when they died, but I couldn't be with my aunt when her time came. I knew though, that when my aunt died she was not alone, I'd been there till two days before and she, too, like your dad, was surrounded by love and kind and caring people.
Take care, thinking of you and sending you strength and courage.
Love, Tina xx