Ah, now I’ve found how to init the plugin – Calendars | Settings | Advanced. That cleared everything.
However I would also point out that when displayed a composite calendar with no associated ‘child’ calendars I got the warning message…
Warning: uasort() expects parameter 1 to be array, boolean given in /var/sites/d/dbc.(redacted).co.uk/public_html/wp-content/plugins/google-calendar-events/includes/feeds/grouped-calendars.php on line 164
Maybe it would be better to give a more explicit warning?
Anyway, great plugin, thanks.