Hi Guys,
Found the answer to my own question (above), so thought I might as well pop it here in case anyone was losing sleep...
On the scenario I had put forward, I was going across the pond and wanted to get the FMC to plot a waypoint at, say, 22W, or 32W - i.e. 2 degrees after the waypoint - to make sure we were on the correct track.
I was trying to insert the waypoint W22, whereas it should have been W022 (i.e. 3 degrees needed for the longitude). It's a built in function of the FMC.
Anyway, there you go. Take it or leave it :-)
NW3