Complex sentence: Originally consisting of one league, Dorset & Wilts 1 split into north and south regional divisions back in 2004 and had even had teams based in Berkshire participating until 2001 when they left to join the Buckinghamshire & Oxon leagues.
Split into simpler sentences: Originally consisting of one league, Dorset & Wilts 1 split into north and south regional divisions in 2004. It had even had teams based in Berkshire participating until 2001, at which time they left to join the Buckinghamshire & Oxon leagues.