Complex sentence: Rylah's political colleagues quickly recognized his talents and by 1953 he was appointed deputy leader of the party the position he was to hold under its leader Henry Bolte for the next 18 years.
Split into simpler sentences: Rylah's political colleagues quickly recognized his talents and in 1953 he was appointed deputy leader of the party. This was the position which he was to hold under the party's leader, Sir Henry Bolte, for the next 18 years.