Complex sentence: In 1962 the company moved to a portable cabin in Swiss Cottage where it remained for nearly 40 years, before, in 2003, the new purpose-built Hampstead Theatre opened in Swiss Cottage.
Split into simpler sentences: In 1962 the company moved to a portable cabin in Swiss Cottage where it remained for nearly 40 years. In 2003 the new, purpose-built Hampstead Theatre opened in Swiss Cottage.