Complex sentence: There was a serious fire in October 1918, which did not affect the boilers or engine and the mill was operating again 10 days later, but the mill frontage was damaged and then rebuilt, though as a single-storey building.
Split into simpler sentences: There was a serious fire in October 1918, which did not affect the boilers or the engine. However the mill front was damaged in the fire but later rebuilt, though as a single-storey building.