Complex sentence: Until 1956 the office of Regius Keeper was combined with the office of His/Her Majesty's Botanist (also established in 1699), and since then the office of HM Botanist has been honorary and conferred on a serving or retired Regius Keeper.
Split into simpler sentences: Until 1956 the office of Regius Keeper was combined with the office of His/Her Majesty's Botanist (also established in 1699). Since then the office of HM Botanist has been honorary and conferred on a serving or retired Regius Keeper.