His decline has been sudden. As recently as six months ago he was still driving, shopping, gardening and going to church, still the same implacable, indestructible alpha male who had always towered over my life.

Then, in late summer, some watershed was crossed. Some tipping-point was reached. Something passed away. To my surprise, he was not immune to the unavoidable consequence of being 91: cell degeneration. A light went out, and where once he bossed and boomed there was only a hollow husk, a frail, vague, distracted, confused old man staring out of the window in his pyjamas. Dad was dead.

But dying isn’t easy. We don’t die on demand. We don’t just switch off. To die takes effort; to die requires work. You’ve got to want it. And, if you’re not suffering too badly, why surrender to an eternity of non-existence rather than plough on for one more day of life, especially as you’re going to die sometime anyway? Why not see another dawn? Why not enjoy another sunset? The Queen Mother, for instance, reached the age of 101 not because of a healthy lifestyle or super-strong genes, but because it’s tremendously motivating to carry on, no matter what indignities decrepitude brings, when you’re a millionaire for whom there will always be a few more worldly pleasures. He’s comfortable, he’s secure, he wants for nothing, he’s not in pain, he seems at peace: so the will to live remains. And that alone is what now sustains him.

For the first time in 50 years, since I was a child, he enjoys my company. I remind him of mum, the love of his life, particularly when we do the Daily Telegraph cryptic crossword in the morning. The newspaper that moulded/endorsed his political opinions for decades is still pushed through the letterbox every day, and he can’t wait to read out the clues to me and then fill in the answers after I’ve worked them out in my head and explained them. “Your mother was good at crosswords. Your mother was the best at crosswords,” he ruminates, almost warmly, in my direction. Our argumentative, antagonistic relationship, always a hair-trigger away from going ballistic and carved into my very bone-marrow across a half-century of textbook Abraham/Isaac conflict, has simply evaporated into thin air. And also gone are all those self-evident truths he held dear which, it turns out, were merely the aggregated propagandas accumulated by most of the conforming masses, and not integral characteristics after all…

He doesn’t criticise me any more. He doesn’t fight me any more. He doesn’t frighten me any more. He has fallen silent. A most unexpected role-reversal has taken place, entirely organically and naturally. Now it’s me who delivers the long, thinking-aloud monologues and he who sits there looking a little bored nodding occasionally. When he does speak it is always of the past – but not his own. His memories are of other people’s memories, particularly his father’s vivid accounts of service in the Royal Navy in World War One – a war which had ended over five years before he was born. This obsession with war is not new, but now it has reached manic levels as he studies maps of the Dardanelles with a magnifying glass, pours over the body count of every Western Front battle virtually soldier by soldier and weeps real tears at the very sight of the Menin Gate as though it were all happening today. “I know it inside out already, dad,” I said rather too snappily the other day, as he cornered me in the kitchen with another urgent, panicky account of the Gallipoli campaign. He looked bemused and upset. I felt guilty. He’s low maintenance really. I must be kinder. He has senile dementia.

Perhaps he will divulge a real memory, a memory he has never properly talked about, a first-hand not second-hand memory, a memory of the moment that defined his life, a memory I’d truly love to hear, but a memory that is obviously so painful even 75 years later that it still cannot be unearthed: the death of his 19-year-old brother in the Battle of the Atlantic in 1940. He has inculcated me with micro-detail about the Cory Brothers cargo ship, Convoy HX-79, German submarine U-46, the U-boat’s commander Engelbert Endrass, the weather, the time the torpedo hit and the precise co-ordinates 90 miles south-west of Rockall where the SS Ruperra now lies at the bottom of the ocean – but has never expressed how he, at age 16, felt when that telegram came from the Admiralty. I’ve stopped asking; perhaps it’s a memory too far.

So we take it one day at a time. This is not what I ever wanted. This is not a task for which I’m temperamentally suited. This is not how I thought events would unfold. But it’s the hand I’ve been dealt. My siblings are doing great, but they live far away. I am now the nearest responsible adult. I must see it through to the end and do the right thing. I must solve the puzzle and complete the grid. That’s what mum would do.