Complex sentence: The Penhydd trail was built by a group of enthusiasts from the local Mountain Bike Club then the Forestry Commission saw how popular Mountain Biking was becoming so the Wall trail was built.
Split into simpler sentences: The Penhydd trail was built by a group of enthusiasts from the local Mountain Bike Club. Subsequently, the Forestry Commission saw how popular Mountain Biking was becoming, so the Wall trail was built.