Complex sentence: The hospital remained a Seaman's Hospital (one of only two offering free treatment exclusively to seafarers) until 1948 and the advent of the National Health Service when it became a general hospital and then a psychiatric facility.
Split into simpler sentences: The hospital remained a Seaman's Hospital (one of only two offering free treatment exclusively to seafarers) until 1948. With the advent of the National Health Service it became a general hospital and then a psychiatric facility.