Andy and ETOPS,
Thanks for those accounts. I thought no-one was going to respond to my comment about changing conditions, and I think it's really important to know that whatever you do, however you plan, things can go wrong. It's one reason why I won't fly unless I'm feeling 100% fit - even a little local flight can turn into something requiring all your knowledge and energy and ability, if you happen to be unlucky. And I've had enough scares - lost in about 3km vis, stranded overnight by a line of thunderstorms (30PROB can happen) - to know it doesn't always happen to other people. You need to be as prepared as you possibly can.
Don't get too scared, but fly safe!