Complex sentence: The Sea King HC.4 gradually replaced the Wessex HU.5 with the Commando Assault Squadrons from 1980 and following the closure of RNAS Portland (HMS "Osprey") in 1999, HMS "Heron" became the main shore base for the Lynx fleet, again.
Split into simpler sentences: The Sea King HC.4 gradually replaced the Wessex HU.5 with the Commando Assault Squadrons from 1980. Following the closure of RNAS Portland (HMS "Osprey") in 1999, HMS "Heron" became the main shore base for the Lynx fleet, again.