Complex sentence: One such error that Brown relates was a then-widely held theory that Churchill had known of German intentions to bomb Coventry in November 1940, but that the British leader had hidden the information to avoid giving up the secrets of Ultra (intercepted German communications).
Split into simpler sentences: One such error that Brown relates was a, then widely held, theory that Churchill had known of German intentions to bomb Coventry in November 1940. But that the British leader had hidden the information to avoid giving up the secrets of Ultra (intercepted German communications).