Complex sentence: The first series premiered on 4 January 2010 on BBC Two, running for 20 half-hour episodes; a further series followed in 2011 where the series length was extended to 25 episodes.
Split into simpler sentences: The first series premiered on 4 January 2010 on BBC Two, running for 20 half-hour episodes. Another series followed in 2011, when the series was extended to 25 episodes.