Thanks for the replies, it does explain a few things.
It seems to me that if the life is 25-30 years, then that's great, so long as there's a coherent plan for replacement kicking in at 25 years, and fully on stream by 30. Invincible was built in 1978, 32 years ago. Illustrious 82, Ark Royal 83. If Illustrious makes it to 2020, it will be nearly 40 years old.
By the sounds of things, she won't, and so there'll be a big old gap with no carriers AT ALL. Mind you, perhaps not so important with no Harriers anyway.