Thanks for the replies guys.
I feel my question above might have been a bit rambling... just to clear up:
On a NAT, you need to make sure you've programmed the FMC correctly, and so having plotted your clearance on a chart, you plot the FMC position 2 degrees of longitude after a waypoint, to make sure you're not in the wrong place.
So - you know where you are at 30W (position checks as 51N 30W or whatever).
The question is where will you be at, say, 32W - you need to check you're not heading off on the wrong course from 51N30W.
The notes I made said that you put 32W or W32 at L1 on the DIR INTC page, creating a waypoint called something like W3201. Bringing this down to the scratchpad would display N5123.4W03200.0 - plot that, then erase the change, and away you go.
NW3