Complex sentence: It was previously rhetorically phrased as the West Lothian Question as an argument against devolution but since the actual act of devolution has now taken place, the onus is upon the need to examine arrangements in England so as to redress the constitutional imbalance.
Split into simpler sentences: It was previously rhetorically phrased as the West Lothian Question as an argument against devolution. Since devolution has now taken place, it is now used to draw attention to the perceived need to redress the constitutional imbalance.