Complex sentence: Several were rebuilt by Drummond's successor, Robert Urie into his new N15 (King Arthur) Class, and as such, the class had been completely withdrawn by the time Richard Maunsell took over as Chief Mechanical Engineer of the newly formed Southern Railway in 1923.
Split into simpler sentences: Several were rebuilt by Drummond's successor, Robert Urie into his new N15 (King Arthur) Class. This meant that the class had been completely withdrawn by the time Richard Maunsell took over as Chief Mechanical Engineer of the newly formed Southern Railway in 1923.