You might be thinking of Malcolm, who was diagnosed with dementia a year or so ago and had to retire because he couldn't play their music anymore. Angus and Malcolm's nephew Stevie has replaced Malcolm. I haven't heard of Angus having any issues—I assume you mean health? For all the devil horns and his naughty-schoolboy image, I've heard he's actually pretty clean-living off-stage.
I knew it was one of them.
Johnson’s book on cars was really good.
Correct, it was Malcolm; and I believe the dementia was the result of a pretty bad stroke. He was a heavy smoker and (until recently) drinker.