Hi John. Thanks for the advice but the problem is that I have unchecked the "all day" box and still some overnight events get displayed as blocks. After some playing around it seems related to the end time, depending how late in the morning I finish decides whether its a block or not. Strange but true.
That does seem strange, but I also see that behaviour. On my OSX 10.8 system, in Month view, if the event finishes at 5am or before it shows as a single day timed event. If after that it shows as 'all day'.
I guess it is designed to display events like parties that do finish after midnight, but may be considered only related to the first date.
This is a user to user forum and Apple don't tend to participate here. If you don't like how the events are displayed, then you can let Apple know: https://www.apple.com/feedback/