Complex sentence: From mid- 2013 onwards, Helder and Griffiths were absent from the band's touring line-up due to other commitments; their positions being temporarily filled by keyboardist Hannah Hird and drummer Alex Cromarty.
Split into simpler sentences: From mid- 2013 onwards, Helder and Griffiths were absent from the band's touring line-up due to other commitments. Their positions are being temporarily filled by keyboardist Hannah Hird and drummer Alex Cromarty.