No amount of pulling/pushing the cable in combination with shifting
the key around seem to help. This is the first time it had really been
used - it's been lying indoors for months so it should have no
corrosion. I sprayed a little PlusGas (non-graphite based) penetrating
oil but this has had no discernible effect.

I know there's not a lot to go on, but does anyone have any
experiences/suggestions for freeing this?

Try spraying a lot of PlusGas? When my lock seized (I broke two of the three keys trying
to get the damn thing to open) I went home and returned with the hallowed WD-40. One spray
and it worked fine. Try drowning your lock in the stuff. If that doesn't work then a very
try a big pair of boltcroppers?

Hope you unlock it before someone else does.

Mark.