Complex sentence: The side was then switched to the Southern League in 2005, where they remained until 2007, but after relegation from the Southern League Premier Division, they were moved "sideways" to the Isthmian League Division One North.
Split into simpler sentences: The side was then switched to the Southern League in 2005, where they remained until 2007. However, after relegation from the Southern League Premier Division they were moved "sideways" to the Isthmian League Division One North.