According to Captain Robert Montgomery (Royal Engineers, attached to No.2 Commando), Campbeltown was meant to have detonated at 4:30am, the delay caused, he believes, by some of the acid in the pencil detonators being distilled away.
Just before the Campbeltown exploded, Sam Beattie was being interrogated by a German naval officer who was saying that it wouldn't take very long to repair the damage the Campbeltown has caused.