Complex sentence: The ship contracted by the UK Government to observe the radio ships off the Thames Estuary (a former sister ship to Laser 558, the'Gardline Tracker') did not return to continue its watch on Radio Caroline.
Split into simpler sentences: The ship contracted by the UK Government to observe the radio ships off the Thames Estuary. A former sister ship to Laser 558, the'Gardline Tracker', did not return to continue its watch on Radio Caroline.