Complex sentence: In 2004, the Next Stage Theatre Company took possession of a grade II listed building originally built as a Congregational hall in 1797, which had been used by The People's Mission until 1998, and began building work to convert it into a theatre.
Split into simpler sentences: In 2004, the Next Stage Theatre Company took possession of a grade II listed building originally built as a Congregational hall in 1797. It was used by The People's Mission until 1998, when building work began to convert it into a theatre.