Hmmm. I tried it here, with 48 rather than 60, and made a new event after restarting iCal - the new event was scheduled to last 48 minutes. You shouldn't need to cancel any previous. What do you get if you give, in Terminal, the command
defaults read com.apple.iCal 'Default duration in minutes for new event'
It DOES read 60.
However, it works now... I am ashamed to admit, but I didn’t do the basic first procedure to fix the problem: restart the computer. Though I quit iCal when doing the Terminal commands, there is probably some setting in the preference file that only gets read once at start-up.
Thank you for your help