Complex sentence: In 1972, NBC and British Leyland's joint project, the Leyland National, became the standard in most NBC fleets, not least LCBS, which took over 500 and ammassed the world's largest fleet of the type.
Split into simpler sentences: In 1972, as part of NBC and British Leyland's joint project, the Leyland National, became the standard in most NBC fleets. London Country, bought over 500 examples and operated the world's largest fleet of the type.