I can't find how to delete my Q but the answer I have now discovered was to delete an all day entry on 1st April from one of my synced calendars. I think it has something to do with summer time ending somewhere the day before and the entry on 1st being all day by reference to times (I.e. 0001 to 2359). I have other entries that day inclu all day entries that simply tick the all day box instead of inserting a time and they do not appear to cause iCal to crash.
Seems this problem has been around since last year and no one has fixed it.