Complex sentence: Cotterill left the NF for the British National Party (BNP) and in 1998 formed and ran the "American Friends of the British National Party" (AFBNP), when he lived just outside Washington DC, which aimed to raise funds for the BNP.
Split into simpler sentences: Cotterill left the NF for the British National Party (BNP). In 1998, when he lived just outside Washington DC, he formed and ran the "American Friends of the British National Party" (AFBNP), which aimed to raise funds for the BNP.