[Neil]

That is an interesting point you raised. The debt should not vanish when your son leaves full time education, not including higher education or reaches the age of 19 as stated in law. He owes you the money from when he was younger so should owe that money forever until he pays it.
The worse thing you can do is not pay. If you return the papers promptly you actually get 3 months free without paying, now that is a kick in the teeth for the parent with care if you ask me.