Complex sentence: Its founder abbot was Wilfrid of York, who was given permission by the kings of Sussex to found it when he was expelled from the bishopric of York in 681, on a site which is now probably submberged by the sea off the Sussex coast near Selsey.
Split into simpler sentences: Its founder abbot was Wilfrid of York, after his expulsion from the bishopric of York in 681. These lands are now probably submberged by the sea off the Sussex coast near Selsey.