Adding UVs to an already unwrapped object

If I UV unwrap 5 sides of a cube, and then realise I’ve screwed up and want to add the 6th face’s UVs, is there a way to do so without resorting to the good ol’ ‘Start Again’ approach?

If there is enough free space on the map, just unwrap the one face and place in position.

Select all the vertices you have in the position you want one your UV, then press “P”, this will pin them to that place on the UV. Then feel free to just unwrap your model again, and all the points should stay in thier appropriate places. This should work if I’ve read what your asking right. (Oh and alt-P unpins selection)

Thanks. That worked.
I’ve only just figured out that the UV view window’s texture (and resulting 3D view window, if you turn the textured display on) have nothing to do with what’s rendered! The texture that appears in the UV view window seems to be a reference piece only.