After serious thinking Chris Green wrote :
I know it's a tall order but still I may as well ask...

Is there any clever way of getting a cable through a hidden stud
behind plasterboard? I want to feed a lamp from a switch mounted on
an internal dry-lined wall but unfortunately there's one piece of
timber between the switch and the light.

It's a very low current requirement, 3.5 watts at 240 volts so
something like 15mA.

If it's near the ceiling, you can drill through, but other than that...

You need to make a hole in the PB, over the level of the stud, get the
cable through, then fill the hole.