Complex sentence: Probably the largest Lady-chapel was that built by Henry III in 1220 at Westminster Abbey, which was wide, much in excess of any foreign example, and extended to the end of the site now occupied by Henry VII's Lady Chapel.
Split into simpler sentences: Probably the largest Lady chapel was built by Henry III in 1220 at Westminster Abbey. This Lady chapel was wide, much in excess of any foreign example, and extended to the end of the site now occupied by Henry VII's Lady Chapel.