Complex sentence: The Germans reported sinking a destroyer, but this was on the basis of the sinking the minesweeper "Bramble" which they mistook for a destroyer--they never realised "Achates" had been hit.
Split into simpler sentences: The Germans reported sinking a destroyer, but this was on the basis of the sinking the minesweeper "Bramble", which they mistook for a destroyer. They never realised "Achates" had been hit.