Thank you for your help.
I have World Adventures, Ambitions, Outdoor Living Stuff and now Pets, that is all. I have no mods and no store or other downloads.
I started a new game after installing Pets. It crashed several times randomly in the first couple of days, but now, it crashes about two minutes after the actual game opens. So, my sim was actually preparing a meal when it crashed and it has now crashed four times at about the same point of preparation, the dog is out foraging, it digs up an item and is returning home. I can't get past this point now.
I surmise that this game is definately currupted so I'm going to try starting another game with similar chaacters and the same lot to see what happens.
If this has the same problems then I think a total re-install and fresh start will be my only option. However I am concerned that I have two competition uploads and an upload in the exchange which will be affected.
I have read all the stickies and can't see anything there which appears to address this problem other than the factory reset and updating of direct X which I have already tried.
Update
I'd like to thank everybody who read this thread, and especially those who tried to help.
My son took a look at the problem a few hours ago. He discovered that my graphics card was only just within minimum requirements and has replaced it with a much better card which he could spare from one of his business computers. I've been running now for several hours without problems, so hopefully I will be OK now though I am saving every 20 minutes or so just in case.
If this is the cure, then it would seem that minimum requirements might, in some cases be inadequate for this game.
Thanks again folks, your time is much appreciated.