Nrfairy:
Thank you for your advice, I don't have an "Advanced Options" button anywhere on Calendar Options so this must be a version difference.
Ridley:
Thank you for all your "fidgeting", hope you didn't waste too much time on it !
The problem manifests itself that my secretary puts in an all day meeting for me, maybe even travelling away, without paying attention to the finish time (!), so it goes in as 0900 to 0930, then someone else does an automatic Meeting Request which says I am free after 0930, then - well you can guess the rest, can't you, it's all my fault !!! So setting the default to all day will stop this happening.