Complex sentence: Wales became a refuge for other Britons following the pagan Anglo-Saxon invasion of what became England, so much so that the Welsh refused to co-operate with Augustine of Canterbury's mission to the Anglo-Saxons.
Split into simpler sentences: Wales became a refuge for other Britons following the pagan Anglo-Saxon invasion of what became England. The Welsh refused to co-operate with Augustine of Canterbury's mission to the Anglo-Saxons.