Nov 16, 2007
Pretty easy actually, you just phone IR and say you're leaving the country, they send you a form for opting out of the tax system and one for continuing NI and you pay between 9 and 12 pounds a month from an offshore bank account (not sure why it moves around, but then again we are talking about the tax authorities who do what they like).
Alternatively, you can opt out of the system completely and redeem your payments to date, otherwise you could be hit with a much bigger NI bill when you return to the country for good, until you pay it you can't use the NHS or claim a pension.
Worth the small financial outlay definitely...