Complex sentence: In 1910 Millwall moved south of the River Thames and although no longer being East London neighbours, the derbies retained their passion; both sets of supporters still consider the other club their main rival.
Split into simpler sentences: In 1910 Millwall moved south of the River Thames and the teams were no longer East London neighbours. Even so, the derbies retained their passion and both sets of supporters still consider the other club their main rival.