Complex sentence: It became apparent that there were no enemy agents transmitting from the UK-in fact, all German agents entering the UK were promptly captured and either interned or "turned" to operate as double agents under the supervision of the "XX Committee".
Split into simpler sentences: It became apparent that there were no enemy agents transmitting from the UK. In fact, all German agents entering the UK were promptly captured and either interned or "turned" to operate as double agents under the supervision of the "XX Committee".