Complex sentence: Rotherham was given command of the HMS "Bellerophon", Known by sailiors as "Billy Ruffian" whose captain John Cooke had been killed in action, and brought her back to Britain, accompanying the HMS "Victory" and Nelson's body.
Split into simpler sentences: Rotherham was given command of the HMS "Bellerophon", called "Billy Ruffian" by the sailors, after its previous captain, John Cooke, had been killed in action. Rotheram brought her back to Britain, accompanying the HMS "Victory" and Nelson's body.