Complex sentence: The Royal Navy considered a mine sweeping version of the SR.N4, hovercraft being almost invulnerable to mines, but it never got further than the concept stage, although an SR.N3 was used by the Inter-Service Hovercraft Unit for trials.
Split into simpler sentences: The Royal Navy considered a mine sweeping version of the SR.N4, hovercraft being almost invulnerable to mines. This never got further than the concept stage, although an SR.N3 was used by the Inter-Service Hovercraft Unit for trials.