It's definitely 2 years - I used to sell car insurance.
Not got a wife/partner who could alternately insure their car in your name and their name?
That would keep your NCB live as it would be used every 12 months, as would theirs.
Alternatively, buy a car for £50 from Autotrader and insure that even though you won't use it - only problem is you'd need somewhere to keep it!