My mum had early dementia, but mercifully she died peacefully before it became real bad. On my visits she'd often tell me that Prince Charles had been to see her, one day it was even Mick Doohan she met on one of her frequent trips to the "top of the mountain", wherever that was, but I'd just ask how they were and if they'd asked after me....which made her giggle and feel good. My dad on the other hand would correct her and say things like "I don't think so" which used to piss me off.....I'd go crook at him and tell him that if she's happy in her own little world then how the fk could that be a bad thing. Then he'd get pissed off with me of course.
Before the big C got him he too was showing very slight signs of memory lapses and confusion.....he took great pleasure in telling me, 2 days after the fact, that this bloke he knew, john, who took him for a ride on dad's bike on his 80th birthday was an excellent rider and a "natural"....he was quite bewildered when I told him that "john" was actually ME.....he couldn't believe it. He was also incapable of wising up to the evil manipulations of my b1tch of a sister..but that's a story for another day.
So yes, dementia is a sad thing, and an even sadder way to depart, especially if the victims had an active and intelligent mind in their healthy days, so I understand exactly where you're coming from Tez. Chin up buddy.