Jon, that's so difficult isn't it? Minty gets bouts of triaditis, inflammation of three organs of the body. It can be painful and needs treatment. The last time was touch-and-go, and cost us nearly £800. He's getting old now, and like you we don't want to put him through being in the vet's for 3 days on a drip, crying for us and trying to grab us with his paw when we visited. Heartbreaking.
However at the moment he's quite perky and has regained his lost weight. But I think the last act of love is to send them on in peace and end the suffering. I must have wept lakes over the years over my lost little mates.
The three Siamese horrors are at this moment in the kitchen munching away on some lovely big lumps of cooked chicken breast. (Well, it is Christmas!)