a year 2000 car is due it`s first nct after 4 years then every 2 after that so it goes 2004, 2006, 2008, 2010 etc.
lets say this car got it`s nct in 04 then it ran out in 06 and the owner either didn`t bother or had the car off the road.
when it gets it`s next nct, say in 07, then it will be a 1 year nct to bring it back in line to the 2 year fixed nct.
if say the car was off the road until 1010 then, providing it passes, it will get the full 2 year nct.
hope this helps