Complex sentence: It was not until the accession of Henry IV in 1399 that England once again had a king who spoke English as a first language (albeit much changed); he was the first since Harold Godwinson, the last Anglo-Saxon king.
Split into simpler sentences: It was not until the accession of Henry IV in 1399 that England once again had a king who spoke English as a first language (albeit much changed). Prior to that, the last had been Harold Godwinson, the last Anglo-Saxon king.