Complex sentence: By the 18th century ships had become larger and Leigh's deep water channel silted up and the town diminished in importance, reduced once more to a fishing village and servicing the London markets via the Thames and by road.
Split into simpler sentences: By the 18th century ships had become larger and Leigh's deep water channel silted up and the town diminished in importance. Reduced once more to a fishing village and servicing the London markets via the Thames and by road.