Complex sentence: The building became a factory and showroom for Beck's British Carnival Novelties for a few years from 1956, then remained empty until in 1978 a campaign began to turn it into a theatre.
Split into simpler sentences: The building became a factory and showroom for Beck's British Carnival Novelties for a few years from 1956, after which it stood empty. In 1978 a campaign began with the aim to turn it into a theatre.