Ok, thanks for the help.
I found that when I deleted the notes completely like you said it didn't crash. I was then able to put the same notes straight back in. Must have been crossed wires somewhere.
I found I had to remove the notes completely or it would crash - in the father's case I left a space (which I didn't notice) - pressing ok caused it to crash. I've just gone back and deleted the space, and this time it didn't crash.
A few people in my tree have "Marriage Notes" in their notes page, but in this version of FTM I only have options for "Personal" and "Research" notes. The marriage notes came in with an imported file. How do I access them or delete them? I've tried going to their marriage page and clicking notes - but there's nothing there.
For future reference, what do you mean by "disconnect them" before deleting people? Normally I just right-click and chose "delete person" - is that asking for trouble?
p.s. I don't understand how this forum works - is there no way to reply to the thread as a whole, or do I have to reply to one of you individually each time?