Fill out the form to tell that you are becoming non-resident and they'll tell you, eventually, what you owe them / what they owe you.

My brother is in tax and he's told me recently that it's no longer a good idea to keep up NI payments after you're non resident as they now take it into account when they are assessing links to the UK when deciding whether your offshore earnings are taxable.

However, as above: you need to take professional advice. Don't rely on what you see on the internet.