Just wanted to post this as I am so sad about it.
Senior Gentlecat is declining slowly - still happy in himself, still stealing crumpets and sleeping on beds and purring, but increasingly slow and stiff, starting to toilet inappropriately. (We changed his food about a fortnight ago to something top end, and I don't think he's managed a solid poo since...grrrrr.)
He's almost 20 and he's had a good life, and I think when the summer turns it will be time to let him go gently. One of the things he still enjoys is sitting on his plank in the pond communing with the frogs, and I don't think he will manage another long cold winter.