Hi Luke,
The report builder cron has a feature that 'locks' the cron while it's running, which is designed to prevent two crons running at the same time. On your system it appears that the cron was 'locked' but never unlocked - this could happen if the cron crashed while processing.
The fix will require a small database change - do you have direct access to the database? If so you could run the following SQL:
UPDATE mdl_config_plugins SET value=0 where plugin='reportbuilder' and name='cron_lock';
If you're not able to run SQL, then let me know and I can put together a file that you can run to fix it.
We are giving this system an overall for the 1.1 release which will prevent this occuring even if the cron crashes.
Simon